Adding authoring doc, PR validation workflow, PR template and definitions/schemas for built-ins

[stuartko]

Prepares the repo for the built-in template specs ecosystem. Includes the following:

• Github workflow for PR validation
  • Obtains a list of repo contributors and blocks PRs from unapproved contributors
• PR template
  • Contributors must provide specific details about the built-in being submitted
  • Contributors must mark off every item on a pre submission checklist
• Built-in template spec authoring doc
  • A document describing the end to end flow for contributing a new built-in template spec/version
  • Explicitly calls out restriction on who can submit built-ins (approved teams/partners only)
• Built-in definition/metadata schemas
• Definition/metadata for Windows VM Quick Start built-in template spec
• Definition/metadata for Shared Gallery Creation built-in template spec