Rust: rust-analyzer update automation - #22590

This PR adds automation for updating rust-analyzer in the Rust extractor.

We:

• Move the human-readable rust-analyzer update instructions to a separate file, such that an agent can read them without reading the entire readme.
• Add a Python script that deterministically runs the fixed steps related to updating the extractor.
• Add some additions to the readme instructions. The step numbers in the readme are aligned with the script.
• Add an agentic workflow. The workflow runs the script and then starts the agent from there. The intention is that we don't waste token having the agent carry out instructions that are simple enough that a script can handle them.

Guide to reviewer:

• The Python script is what I consider the most valuable part of this PR – so I think focusing on that makes sense.
• The changes to the instructions are best seen by the diff in 04e44e9.
• The agentic workflow is entirely AI generated. I've skimmed it and confirmed that it looks reasonable, and I'd expect a reviewer to do no more. I have no way of running it locally. My understanding is that since it's a new workflow we'll have to merge it before we can run it. I think we should just merge it, and then I'll fix stuff in follow up PRs if it turns out to not work.