Add automated interface generation for Payments contract

[pali101]

Summary

This PR introduces an auto-generated IPayments interface for the Payments contract, along with tooling and CI checks to ensure it remains consistent with the contract ABI.

Changes

• Generated src/interfaces/IPayments.sol using abi-to-sol for external use and inheritance
• Script: tools/generate-interface.sh builds the contract, extracts its ABI, and generates a clean IPayments interface
• Refactor: Moved IValidator to src/interfaces/IValidator.sol
• Makefile: Added make interface target
• CI: GitHub Action to check interface drift on push/PR to main
• Docs: Updated tools/README.md with usage details for the script and make interface

Closes #122

[rvagg]

optional, but you could pull these two versions up to variables at the top of the file so we can update it more easily, but it's not a big deal, we'll probably only ever change this when stuff doesn't work the way we expect -- unless we have the solidity version hard-wired somewhere else too?

[aarshkshah1992]

@pali101 Let's keep this PR open till we integrate it with Pandora and make sure the interface/compilation/tests work in Pandora as well.

[rvagg]

lgtm if it all works nicely locally (sorry I haven't had time to actually try it out myself)
I'll stop tormenting you now; good work!

[rjan90]

It seems like switching to interfaces did not have any impact on contract size, ref conversation that happenend in Slack here:

Switching to interface didn't have any effect on contract size. Unless we inherit it, it doesn't matter if we utilize contract or it's interface to call its functions.
Reference: https://ethereum.stackexchange.com/a/136871/67618

Do we expect that this PR will land, or should it be moved back to draft?

[rjan90]

From @aarshkshah1992 in Slack: It didn't get us the size reduction we needed. Now, there's no urgency here at all.

I will revert this back to draft for now, to signal that this is not expected to land until the audit is completed 2025-08-11