[java] Improve pom.xml to qualify for publishing to Maven central (#4616)

* [java] Attach Javadoc to artifact generation.

* [java] Attach source to artifact generation.

* [java] Add gpg signing to artifact publishing.

* [java] Add artifact URL to pom.xml .

* [java] Add artifact description to pom.xml .

* [java] Add artifact URL and description params to Jax RSS.

* [java] Add developer info to pom.xml .

* [java] Parameterise SCM info in generated pom.xml .

* [java] Move GPG signing to verify phase so that .asc files are uploaded during deploy phase.

* [java] Change GPG signing to be an optional via Maven profile. Can't assume all users will perform a release/deploy from an environment with correct GPG key/pass.

* update java petstore smaples

* camelize tag name, remove invalid file

* add back missing files for okhttp-gson

* fix docstring in java feign client

* fix docstring with various java api clients

samples/client/petstore/java/okhttp-gson-parcelableModel/docs/MapTest.md

@@ -12,6 +12,8 @@ Name | Type | Description | Notes
 ## Enum: Map<String, InnerEnum>
 Name | Value
 ---- | -----
+UPPER | "UPPER"
+LOWER | "lower"