offers: add used_count to listoffers

[alexgrad42]

This adds a new 'used_count' field to the 'listoffers' RPC command output to see the exact number of times a BOLT12 offer has been paid by counting associated settled invoices.

Changelog-Added: add a new 'used_count' field to the 'listoffers' RPC command

Closes #9146

[nGoline]

Great addition. This fills a real gap and the implementation is straightforward, the core idea is solid and it's close to being mergeable.
A few things to fix before we can land this.

[nGoline]

Please rebase onto master instead of merging it in. Merging master into your feature branch creates a noisy history and makes the diff harder to follow. A clean rebase keeps the commits linear and makes it easier to bisect if something goes wrong later

[nGoline]

Please rebase onto master instead of merging it in. Merging master into your feature branch creates a noisy history and makes the diff harder to follow. A clean rebase keeps the commits linear and makes it easier to bisect if something goes wrong later

Also, when addressing the review feedback, please use fixup! commits (or git commit --fixup=<sha>) rather than separate "fix" commits. Having a commit like "fix indentation" or "address review comments" in the final history makes it harder to bisect.

[alexgrad42]

I managed to squash the commits right before realizing that you prefer separate fixups, sorry about that! I will definitely use git commit --fixup next time.

[nGoline]

I was talking to @rustyrussell last week and he mentioned the sql plugin. It was specifically designed to avoid baking ad-hoc aggregations into individual RPCs.

Following the philosophy of keeping individual RPCs lean, I strongly believe the sql plugin already covers this use case without adding a field to listoffers that has to be maintained, schema'd, and kept in sync across gRPC, msggen, etc.

Thank you for the time you've put into this @alexgrad42. I'll add instructions on how to achieve the proposed functionality on the original issue (#9146)