[Ruby] Add support for populating a gem metadata

[ivgiuliani]

Currently there is no easy way to specify a gem's metadata. Metadata is often used to reference related links for a given gem, but is otherwise a generic key/value dictionary that can be looked up by consumers of the gem. For example, can be used to specify if multi factor authentication is required when publishing on rubygems (rails example).

This pull request adds support for populating a gem's metadata by introducing an additional property (gemMetadata). This can be any arbitrary object, even though it should follow the rubygems specs.

I briefly considered using a list of entries instead where each entry would be a key/value pair in the hash. However the somewhat arbitrary nature of values (urls, strings, bools, ...) make the implementation quite complex with very little return of investment, given that complex cases with this PR can be easily worked around by just using a multiline yaml in the config file. E.g. something like this produces the correct result:

additionalProperties:
  gemMetadata: >
  {
    "bug_tracker_uri"   => "https://google.com",
    "rubygems_mfa_required" => "true",
  }

Fixes #16870

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

LGTM. Thanks for the PR.