Change branded setting to "flavor"

[timovv]

Fixes #2324

• Add new option flavor. When set to Azure, an SDK using Azure Core will be generated.
  • flavor defaults to Azure if the package scope starts with azure or msinternal (for tests). Eventually maybe we should make our tests emit unflavored code by default and only select Azure flavor where necessary.
  • For backward compat, if branded is set to true and flavor is left unset, flavor will be set to Azure.
  • Otherwise, if flavor is unset, or is set to any other value, an unbranded package will be generated.

[joheredi]

Looks good, thanks for taking this work! 🚢

[MaryGao]

Thanks for this! I approved and just left small comments.