Create signer_bug Issue Template

[will-corcoran]

Adding an issue template for Signers to submit bugs and issues with the Signer

Description

Applicable issues

• fixes #

Additional info (benefits, drawbacks, caveats)

Checklist

• [ ] Test coverage for new or modified code paths
• [ ] Changelog is updated
• [ ] Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• [ ] New clarity functions have corresponding PR in clarity-benchmarking repo
• [ ] New integration test(s) added to bitcoin-tests.yml

[BowTiedDevOps]

Also, the file needs to have a markdown extension:

- .github/ISSUE_TEMPLATE/signer_bug
+ .github/ISSUE_TEMPLATE/signer_bug.md