`CodeSigningRequest` Starlark type should expose existing signing information

Filing this issue to track a known missing feature of code signing where the existing code signature metadata should be exposed.

This metadata can be useful for filtering signing requests in signing callback functions. The most obvious case is wanting to ignore signing a file multiple times.

Locate the CodeSigningRequest Starlark type and the signing callback entry points, then trace where existing code-signature metadata is available. Done means signing callbacks can inspect that metadata and use it to filter requests, including avoiding repeated signing of a file.

Written by the indexing model from the issue text.