Add SBRP tooling to support text-only packages

[MichaelSimons]

Tooling should exist to automate the process for adding a text-only package to SBRP. User should be able to provide the package name and version with an optional feed. The tooling in dotnet/installers around text-only package generation may be a good reference for implementing this.

[oleksandr-didyk]

Additional information:

• existing tooling in dotnet/installers can be found here
• entry point for the resulting tooling should be the ./generate.sh in SB repo root (source)
• usage of the prepared mechanism should be documented at least in the source build docs (available here)

[oleksandr-didyk]

Update on progress -> started migrating MSBuild logic found in dotnet/installers to SBRP; in general, the idea behind the solution is to have an MSBuild project similar to that in dotnet/installers, with the small difference that the package name, version and optionally feed are provided as CL properties provided by the entrypoint script

[oleksandr-didyk]

Update on progress -> during PR review it was decided to extend the current task goal with merging the existing reference package generation tooling and the proposed text-only one for better maintenance / re-usability; currently working on adding that to the opened PR

[oleksandr-didyk]

Update on progress -> PR feedback incorporated into the branch, waiting for re-review