Support a build identifier for identifying different builds of the same (code, rev, path, language) tuples

[varungandhi-src]

Specifically, if one has an x86_64 Linux build and arm64 macOS build, it would be useful for code nav data to include the build identifier. Thoughts on how this build identifier would be used etc.

1. The build identifier would be a new string field in SCIP, not a part of src-cli. That way any tool inspecting an index has access to the build identifier.
2. The build identifier would be used to identify when an upload overrides another upload.
3. The build identifier would be exposed in the UI (maybe only for repos which have non-empty build identifiers?). This would allow a user to select the build in some way, and then code intel requests from the frontend to the backend would also include the build identifier (in addition the the revision etc.).