Add generator Tools release and refresh

[Alancere]

• 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.

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[RickWinter]

Is there a document describing the tool workflow? Also I'm interested to learn more about why Go was picked as the language to write this automation. The central EngSys team uses powershell for scripting and automation.

[Alancere]

This is migrating the remaining commands from the private tool to public repo (track1).

[ArcturusZhang]

Is there a document describing the tool workflow? Also I'm interested to learn more about why Go was picked as the language to write this automation. The central EngSys team uses powershell for scripting and automation.

These tools are only for track 1 SDK generation. Previously we have a specific private tool built for generating new releases for track 1 SDK to avoid too much manual work. We are trying to move that tool to this public place so that our customer could also try it if they want to generate their own private release of track 1 SDKs.

We are also building new tools for track 2 releases as well.
We understand we should be keeping everything in eng directory and write things in powershell, but there are things we do not find a powershell equivalent yet (for instance, generate changelog for the SDK packages). Therefore we are also implementing that in go in the tools/generator directory. But we will migrate that to powershell and eng directory after we get the prototype running.

FYI the similar PR for track 2 is here: #15271 (review)

[ArcturusZhang]

Please goimports all of the new files