feat: add librarian action

[noahdietz]

Adds a root-level custom action for installing librarian based on the version specified in the triggering repo's librarian.yaml or latest. The version specified in the uses directive e.g. @main is the GitHub ref to use in pulling the custom action definition, not the version of librarian the action would ultimately use.

- name: Run librarian tidy
uses: googleapis/librarian@main
run: librarian tidy

Fixes #5683

[gemini-code-assist]

Code Review

This pull request introduces a new composite GitHub Action to set up Go, protoc, and the Librarian tool. The review highlights the need for more robust version parsing in the installation script and suggests referencing the action's own go.mod file to ensure the correct Go version is used regardless of the caller's repository structure.