Add eNear to the omni bridge

[kiseln]

(Not planning to merge this yet because of conflicts with #74)

Added a standard to map custom Ethereum tokens to the Near counterparts. This is useful to migrate existing (legacy) rainbow bridge assets to the OmniBridge. The first implementation is for eNear.

Created a proxy (or adapter?) for eNear that will be the only address that can mint/burn eNear tokens.

• For burning. We have to transfer the tokens to the proxy address and then call eNear transferToNear to burn.
• For minting. We set a fake prover that always returns true. We submit a fake proof with appropriate amount and recipient to eNear's finaliseNearToEthTransfer.

I think I should move eNear implementation to this repository and add tests.