Add optional bundle dependency

Hello,

I am using bundle `com.fasterxml.jackson.jaxrs.jackson-jaxrs-json-provider` in an OSGi environment. The first thing I got was this exception:

> java.lang.ClassNotFoundException: com.fasterxml.jackson.module.jaxb.JaxbAnnotationIntrospector cannot be found by com.fasterxml.jackson.jaxrs.jackson-jaxrs-json-provider_2.6.3
> at org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:432)
> at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:345)
> at org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:337)
> at org.eclipse.osgi.internal.loader.ModuleClassLoader.loadClass(ModuleClassLoader.java:160)
> at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
> at com.fasterxml.jackson.jaxrs.json.JsonMapperConfigurator._resolveIntrospector(JsonMapperConfigurator.java:109)

The bundle `com.fasterxml.jackson.module.jackson-module-jaxb-annotations` was active, but not visible by the class loader of the first bundle.

My workaround was a fragment bundle. However, my suggestion here is to make the first bundle depend _optionally_ on the other:

```
Require-Bundle: com.fasterxml.jackson.module.jackson-module-jaxb-annotations;bundle-version="2.6.3";resolution:=optional
```

Thanks for the good work,
Björn