Add generic signed message method for verifying POAPs

### User Story

My POAPs are not in a wallet directly integrated with my browser, so I want to be able to sign and submit a generic message in order to verify them for gitcoin purposes.

### Why Is this Needed

*Summary*: The existing flow for verifying POAPs assumes that the user stores their POAPs either right in the browser or in a wallet which can connect directly to the browser. However the signed message standard used for the verification is supported by a much wider variety of wallets. If there was a generic message signing flow all of this other wallets, some of which may be offline, in cold storage, etc. could be easily supported.

### Description

*Type*: Feature

### Current Behavior

Currently a wallet must be directly connected to the browser to verify a POAP.

### Expected Behavior

I would like to directly input an address, be prompted with a plain text message to sign, and be offered a place to paste the signed message for verification.

### Definition of Done

Users can sign a message in any wallet that supports message signing to verify their ownership of a qualifying POAP.

### Data Requirements

Nothing different than the existing POAP verification flow.

### Additional Information

None