[Bridges SDK] Open Market Decentralized Relayers

[acatangiu]

Follow-up on the work in Coordinated relayers protocol for cheaper relaying.

Aside from the Parity work linked above, Darwinia have also implemented something similar:

• https://medium.com/darwinianetwork/darwinia-apps-guide-part-v%E2%85%B0-fee-market-analytics-9cd909e9e8
• https://forum.polkadot.network/t/discussion-bridging-assets-via-external-bridges-to-different-parachains-via-xcm/1660/15

End goal of this epic is to have some permissionless open-market Relayer coordination protocol to increase decentralization while keeping fees at fair market prices regardless of various tokens price action.

Follow-up epics to integrate this into P<>K and P<>E trustless bridges.