[TypeSpec Requirement] Add awareness of "service groups"

[mikeharder]

https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/directory-structure.md#advanced-scenario-service-group

Example PR adding a sub-service:

https://github.com/Azure/azure-rest-api-specs-pr/pull/17717/checks?check_run_id=24311732426

A possible fix would be, if we don't find specification/foo/resource-manager/Microsoft.Foo/Bar/stable, we would fallback and look for specification/foo/resource-manager/Microsoft.Foo/stable. If this parent stable folder exists, pass the check.

[konrad-jamrozik]

@mikeharder what is the allowed structure of service groups is not yet well-defined. Currently I have a PR open where I try to clarify this:

https://github.com/Azure/azure-rest-api-specs/pull/28929/files#diff-5b72db59365e9ce8cfc776cd9a1437ebba24e85e4dada20b5a7835aa39e97e22R245-R252

However:

• Ideally this proposal would get stamp of approval from @JeffreyRichter and @rkmanda
• This proposal may require tweaking / expansion to accommodate more allowed scenarios.
• We probably we will need to support in the tooling service groups that already are deployed and violate the allowed structure.
• Currently I do not have good intuition on how varied the service group folder structures are, i.e. how many different cases we have to handle, and how many of them will be considered allowed going forward, vs. deprecated-but-still-supported.