ci: automate weekly cargo-rbmt update PRs

[satsfy]

Like rust-bitcoin, corepc could bump rbmt automatically. Currently, rbmt version is at 0.2.0 while rbmt is at 0.5.0, and I didn't notice until I tried using rbmt 0.4.0 feature thanks to this message.

Honestly, this adds maintenance burden and may be too marginal and unnecessary for corepc, but I opened the issue anyway so we can track it.

The workflow can be copied from rust-bitcoin, and the PAT can be stored as a repo secret for the create-pull-request action, because PRs opened with the default GITHUB_TOKEN do not trigger CI runs.

[tcharding]

I don't think we should be doing this automatically. cargo rbmt is moving too fast the updates have loads of stuff in and need manual consideration IMO. Even when reviewing I do so only lightly and trust that Nick knows whats going on. He is writing it with AI and this has been the review process, for me at least, for some time.