fix: add handler for new attachments

[matteojug]

Description

The sBTC signers stalled their Stacks nodes in prod when the nodes emitted "attachment" events (which is a known bug on the node-side), and the sBTC signers couldn't handle it.

Changes

This adds a dummy endpoint responding with 200 OK.

Testing Information

Add simple test, and tested that everything else still works via devenv demo script.

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[djordon]

Looks good