Alternate message triggers for the relayer

[nambrot]

Currently, relayers look to process messages after indexing them on the origin chain mailbox they observing. However, there may be other "messages" relayers could be processing with minor-ish modifications. This ticket tracks such messages, and what they could look like

• In #2398 , @AlexBHarley introduced a ChainlinkISM which is able to process oracle reports on a particular destination chain. Currently, the construction still requires the triggering of an artificial message.
• Alternate origin state proving mechanisms a la (https://github.com/hyperlane-xyz/hyperlane-monorepo/issues/2443) would also require relayer changes to know what to index (instead of Dispatch events on the Mailbox

In some regard, this becomes a generalized relayer, akin to Chainlink Automation or Gelato and we should be careful to not overly complicate the relayer and thus make it worse at its main job of relaying messages between chains.

### Tasks
- [ ] https://github.com/hyperlane-xyz/hyperlane-monorepo/issues/2443

[nambrot]

Telepathy/Succinct folks probably had to think about this, so we can probably learn from their thinking:

https://hackmd.io/@succinctlabs/rywE5pam2?type=view

I.e. require somebody to create a "subscription" which is really just a message trigger