[java][native] Remove JAX-RS dependency from generated java-native client.

[shartte]

This removes the dependency on JAX-RS from the Java native client template by making the following changes:

1. Remove use of GenericType
   This was originally introduced in [Java][Native] Support oneOf/anyOf schemas #7263 by presumably copy pasting a piece of code for OneOf/AnyOf support from the Jersey client. It was using GenericType from JAX-RS, which seems unnecessary since it doesn't seem to generate generic type instantiations (the anyOf/oneOf deserializer will also use {{.}}.class, and not GenericType or similar type tokens).

2. Remove ContextResolver implementation
   The JSON class implements the JAX-RS ContextResolver interface, which seems unnecessary for a native Java client in a non JAX-RS environment (probably copy paste)

3. Removed the respective dependencies from build.gradle/pom.xml.

4. [OPTIONAL] Set source encoding to UTF-8 in build.gradle
   I had to set the source encoding in the build.gradle template, as the samples were not even building for me otherwise.
   The pet store API spec seems to contain UTF-8 characters which are copied into the source, breaking the build with windows default encoding for me. (This should probably be split into a separate PR; but I was unable to build and validate the sample without this fix).

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[wing328]

cc @bgong-mdsol (the author of #7263)

[shartte]

@wing328 Since there has been no feedback in 2 weeks from @bgong-mdsol, can someone else take a look at this?
Additionally: The native client is not supposed to have a JAX-RS dependency, right? (that's what the JAX-RS client seems to be for).

[wtrocki]

Would that also fix other libraries like --resteasy? I still see output having GenericType