Index on-chain message signatures

[Uxio0]

What is needed?

Onchain message signatures should be indexed, same as we do for MultisigTransaction signatures.

Background

• Offchain signatures were implemented on Support EIP-1271 offchain signatures #1114
• Some Safes combine offchain and onchain signatures, so for it to work we need to index on chain signatures also.

Proposed solution

• For non L2 networks: process signMessage calls: https://github.com/safe-global/safe-contracts/blob/ee92957307653ae6cf7312bbcb1a13c6884ea6ea/contracts/libraries/SignMessageLib.sol#L20
• For L2 networks: process SignMsg event: https://github.com/safe-global/safe-contracts/blob/ee92957307653ae6cf7312bbcb1a13c6884ea6ea/contracts/libraries/SignMessageLib.sol#L23
• Allow SafeMessage message to be optional, as when indexing we only have the hash and no way to know the message for it.

Questions

• Currently SafeMessage needs to have at least one signature to be POSTed to the service. Should we allow SafeMessage with no signatures so when there's an onchain signature message can be shown?