[Generator] Refine metadata and changelog

[ArcturusZhang]

Now the generator tool will also generate the corresponding _meta.json file.
Refactor the generator tool, export some functions so that other tools could consume.
Refine changelog format.
Support SDK automation pipeline: add breaking change items and version.

• The purpose of this PR is explained in this or a referenced issue.
• The PR does not update generated files.
  • These files are managed by the codegen framework at Azure/autorest.go.
• Tests are included and/or updated for code changes.
• Updates to CHANGELOG.md are included.
• MIT license headers are included in each file.