Add support for EIP-712 typed data signing in livepeer_cli

[yondonfu]

What does this pull request do? Explain your changes. (required)

This PR adds support for EIP-712 typed data signing in livepeer_cli.

Prior to this PR, the "Sign message" option in livepeer_cli sent a request to the /signMessage endpoint on the node's server in order to sign an arbitrary user provided message. The L1Migrator contract for the Confluence LIP allows signature based authorization, but uses the more recent EIP-712 signing standard which describes a standard way to sign the hash of structured data.

Specific updates (required)

In order to support EIP-712 signing in livepeer_cli I've made the following updates:

• Upgraded the go-ethereum dep in order to use the latest package APIs for EIP-712 signing
• Added a SignTypedData() method to complement the existing Sign() method in the eth.AccountManager implementation
• Added support for requesting EIP-712 signing in the /signMessage handler by setting a SigFormat header
• Added a "Sign typed data" option in livepeer_cli to complement the existing "Sign message" option

How did you test each of these updates (required)

Updated unit tests and manually tested EIP-712 signing with livepeer_cli.

Does this pull request close any open issues?

Fixes #2141

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[darkdarkdragon]

small typo, otherwise LGTM