Provide ability to plug in pre-configured ObjectMapper

[sammyhk]

Currently configuring ObjectMapper can be either:

1. implement ObjectMapperConfigurer which is in decorator pattern, cannot use the existing ObjectMapper configuration used in other part of the system.
2. implement ObjectMapperProvider to create/ re-use existing ObjectMapper configuration, but this way may have code coupling on ConfiguringObjectMapperProvider because current implementation some configuration is hard-coded in ConfiguringObjectMapperProvider (https://github.com/graphql-java-kickstart/graphql-java-servlet/blob/master/src/main/java/graphql/servlet/ConfiguringObjectMapperProvider.java#L22-L28), new implementation need to duplicate those code in order to make it work (e.g. issue Avoid explicitly inject ObjectMapper to the ObjectMapper #177)

Suggested improvement:

1. move those hard-coded configuration in ConfiguringObjectMapperProvider to DefaultObjectMapperConfigurer, others would like to implement ObjectMapperConfigurer can either extend DefaultObjectMapperConfigurer to keep default configuration (with some extra configuration) or implement its own by directly implement ObjectMapperConfigurer.
2. ConfiguringObjectMapperProvider optionally accept a pre-configured ObjectMapper as the base, then call ObjectMapperConfigurer to decorate it.