[keyserver] Introduce isValidEthereumAddress

Summary:
Simple RegEx sanity check to ensure that the address is a well formed ethereum address.

Use utility in siweAuthResponder to throw a ServerError if address fails isValidEthereumAddress check. (Will be used in isValidSIWEMessage instead.)

Depends on D6028

Test Plan: Unit tests and tried calling endpoint with correct and incorrectly formed addresses.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6029