Add plugin for Peerswap atomic swap based local liquidity management protocol

[remyers]

This PR implements the draft PeerSwap protocol as an Eclair Plugin and builds off of PR #2342 with a cleaner commit history after reorganization into a plug-in. This PR requires PR #2495 which makes some prerequisite changes to Eclair.

The plugin creates a SwapRegister actor at startup which will spawn swap sender actors in response to user CLI commands. The SwapRegister will also spawn swap receiver actors in response to swap requests received from peers. The SwapRegister forwards swap messages received from peers based on their swapId field to corresponding local swap actors. When a swap actor has completed, it simply stops and this triggers the SwapRegister to remove it from its map.

The Swap database check-points information about in-progress swaps after a swap maker has committed an on-chain tx or a swap taker has initiated an off-chain payment. Details about the result of the swap are added to the swap record when it completes.

Protocol implementation TODOs:

• add CLI and/or configurable "allow-list" for swap peers and other limits for accepting swap requests
• channels should fail proposed htlc updates that would remove liquidity needed for an active swap
• add or adjust timeouts and timeout behavior to ensure safe operation of the protocol
• re-enable the logic to check for acceptable premium values (see PR #151)

PeerSwap specification suggestions/questions to discuss:

• custom byte encoded message rather than json #1
• use 32-byte channel ids instead of short channel ids #2
• nit: rename “OpeningTxBroadcasted” to grammatically correct “OpeningTxBroadcast” #3
• send a signature instead of a private key for cooperative closes #4

[remyers]

The comment from @t-bast about making Maker/Taker actors use SupervisorStrategy.resume is addressed by d272226. This commit also adds comments to explain the choice.

The change to resume has been made safe by checking that no swap has been saved as a checkpoint to the swap db before the Maker funds the opening tx or the Taker pays the swap invoice.

Should I keep the logic from 036e7f6 that I added to handle the case where a Maker or Taker stops prematurely before recording a result?

[remyers]

Fixed to remove generic messages as per the comment from @t-bast in c2c8460.

[t-bast]

I have created the eclair-plugins repository, it would make sense to move this work over there: https://github.com/ACINQ/eclair-plugins

[remyers]

This PR is continued in the eclair-plugins repository as PR #5