feat(gateway-contracts): introduce the custodians as part of the backup feature

[isaacdecoded]

This PR only introduces the custodians' metadata and initialization. As part of the backup feature, there are other incoming implementations regarding the different phases of the backup logic:

1. Setup/Update custodians phase (implies a KMS context refresh)
2. Key backup phase (receives signatures and emit backup completed event)
3. Recover shares phase (requires multi sig)
4. Dry run phase

Closes zama-ai/fhevm-internal#137

[melanciani]

looks great ! just have one question for the MPC team to clarify before we can merge this

[melanciani]

actually, regarding what we've discussed in #142, we now would need to consider versioning the contracts and anticipate upgrading them

[melanciani]

sorry, a few things has changed, in the end :

• https://github.com/zama-ai/fhevm/issues/247 will need to be done before
• as updated in https://github.com/zama-ai/fhevm/issues/222, we actually won't consider multiple files for different versions, only the latest one

[melanciani]

also please update the test suite and make sure sure e2e tests pass with your changes

[melanciani]

sorry, realized we are missing a safeguard regarding updates, better to consider it before continuing this PR imo : https://github.com/zama-ai/fhevm/issues/261

wdyt ?

[melanciani]

thanks ! I spotted a few missing details + you might want to merge the upgrade PR + rebase

[isaacdecoded]

thanks ! I spotted a few missing details + you might want to merge the upgrade PR + rebase

Missed methods and tests for custodians pushed in the last commit 🙏

[melanciani]

alright, thanks a lot !!