Support method injection of JSON testers

Something like this:

@JdbcTest
@AutoConfigureJsonTesters
class SampleTests {

    @Test
    void sampleTest(@Autowired BasicJsonTester jsonTester) {
        assertThat(jsonTester.from("{}")).hasJsonPath("$");
    }

}

Fails with:

Uninitialized BasicJsonTester
java.lang.IllegalStateException: Uninitialized BasicJsonTester
	at org.springframework.util.Assert.state(Assert.java:80)
	at org.springframework.boot.test.json.BasicJsonTester.verify(BasicJsonTester.java:166)
	at org.springframework.boot.test.json.BasicJsonTester.from(BasicJsonTester.java:110)
	at com.example.SampleTests.sampleTest(SampleTests.java:17)

Looking at JsonTestersAutoConfiguration and JsonMarshalTestersBeanPostProcessor, only fields are considered for tester initialization:

https://github.com/spring-projects/spring-boot/blob/10e2f34eccb1152c9a8259f64d2d5c0456288892/core/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/json/JsonTestersAutoConfiguration.java#L74-L78

This prevents consistent code style in codebases where method injection is preferred.