Enable signing in the VMR official pipeline

[mmitche]

Enable the signing infra to function in official VMR pipeline. This means getting the pipeline approved for signing, Ensuring that the microbuild plugins are installed, etc.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ellahathaway]

T-Shirt Size: XS

Part of this work is handled by other issues (see #4678).

This should involve ensuring that the proper microbuild plugin is installed given the agent OS and that -sign is passed (done in dotnet/sdk#44306)

[ellahathaway]

Link to changes (currently internal due to testing in the pipeline)

This is experiencing multiple blockers; the blockers are being tracked below.