Support EIP-712 signing

[yondonfu]

The current Confluence L1Migrator contract supports authorization via a EIP-712 signature.

At the moment, the livepeer_cli does not support EIP-712 signing. However, there is logic in clef

There should be a "Sign typed message" option in livepeer_cli. When this option is selected:

• The user can paste in the typed data to sign in JSON form. The typed data should follow the format described here for the data arg
• livepeer_cli will send the data to a /signTypedMessage endpoint on the livepeer server similar to the /signMessage endpoint
• The server will convert the data to an instance of apitypes.TypedData
  • See these tests for reference
• There can be a SignTypedMessage method defined in the eth.AccountManager similar to the Sign method except it is responsible for producing EIP-712 signatures
• The implementation of this method can be based off of this method
  • At first glance, does not look like we can import this method directly so next easiest thing may be to pull that logic into go-livepeer

Alternative

We could integrate with clef, geth's external signer. I like the idea of separating the signer codebase from the rest of the go-livepeer codebase. But, this is a bigger change to architecture that I think is better done separately. For this issue, I think we should find the shortest path to supporting EIP-712 signing with minimal change to UX.