Spring Boot 中使用自定义keyConverter，DefaultSpringKeyConvertorParser类中当执行parseKeyConvertor方法时applicationContext为null

环境：
Spring Boot：2.3.9.RELEASE
jetcache：2.6.0

Spring Boot部分配置：
```
jetcache:
statIntervalMinutes: 15
areaInCacheName: false
local:
default:
type: caffeine
keyConvertor: myKeyConvertor
limit: 100
remote:
default:
type: redis.springdata
keyConvertor: bean:myKeyConvertor
valueEncoder: java
valueDecoder: java
keyPrefix: tecache
```

使用默认的keyConverter：fastjson没有问题
当使用自定义myKeyConvertor（此bean已在Spring Context中定义）时，启动应用后，当程序执行到DefaultSpringKeyConvertorParser类中parseKeyConvertor方法时applicationContext为null，报java.lang.NullPointerException，
stack信息如下：
```
Caused by: java.lang.NullPointerException
at com.alicp.jetcache.anno.support.DefaultSpringKeyConvertorParser.parseKeyConvertor(DefaultSpringKeyConvertorParser.java:28) ~[jetcache-anno-2.6.0.jar:?]
at com.alicp.jetcache.anno.support.ConfigProvider.parseKeyConvertor(ConfigProvider.java:90) ~[jetcache-anno-2.6.0.jar:?]
at com.alicp.jetcache.autoconfigure.AbstractCacheAutoInit.parseGeneralConfig(AbstractCacheAutoInit.java:75) ~[jetcache-autoconfigure-2.6.0.jar:?]
at com.alicp.jetcache.autoconfigure.ExternalCacheAutoInit.parseGeneralConfig(ExternalCacheAutoInit.java:19) ~[jetcache-autoconfigure-2.6.0.jar:?]
at com.alicp.jetcache.autoconfigure.RedisSpringDataAutoConfiguration$SpringDataRedisAutoInit.initCache(RedisSpringDataAutoConfiguration.java:65) ~[jetcache-autoconfigure-2.6.0.jar:?]
at com.alicp.jetcache.autoconfigure.AbstractCacheAutoInit.process(AbstractCacheAutoInit.java:68) ~[jetcache-autoconfigure-2.6.0.jar:?]
at com.alicp.jetcache.autoconfigure.AbstractCacheAutoInit.afterPropertiesSet(AbstractCacheAutoInit.java:49) ~[jetcache-autoconfigure-2.6.0.jar:?]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1858) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1795) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:311) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.resolveBeanByName(AbstractAutowireCapableBeanFactory.java:453) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:527) ~[spring-context-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:497) ~[spring-context-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:650) ~[spring-context-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:228) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessProperties(CommonAnnotationBeanPostProcessor.java:318) ~[spring-context-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1425) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657) ~[spring-beans-5.2.13.RELEASE.jar:5.2.13.RELEASE]
... 21 more
```

Start with DefaultSpringKeyConvertorParser.parseKeyConvertor at line 28 and trace how applicationContext is initialized through ConfigProvider.parseKeyConvertor and AbstractCacheAutoInit.parseGeneralConfig. Reproduce the failure with the shown Spring Boot configuration and custom bean, then verify startup completes without a null-pointer exception and the custom key converter is resolved.

Written by the indexing model from the issue text.