[WIP] Integrate BrainParameters with ActionSpec, update BrainParametersDrawer

[dongruoping]

Proposed change(s)

• Add an ActionSpec in BrainParameters, replace old fields in BrainParameters with corresponding fields in ActionSpec and deprecate the old fields
• Add serialization callbacks to propagate ActionSpec fields from the old fields
• Update BrainParametersDrawer to allow specifying hybrid actions in editor
• ActionSpec is modified to be not read-only

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

JIRA: MLA-1320

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[dongruoping]

closing this and replacing with #4718.