Adding a get_signer function to the Auth module

[nfurfaro]

This adds a wrapper around the ec_recover_address() function. Completing this implementation properly requires:

• refactoring the ec_recover functions to return Results
• the ability to use match with enums so we can act according to the specific Result variant returned from ec_recover_address()

This adds feature required for FuelLabs/sway#1061

[nfurfaro]

@adlerjohn This is targeting item number 2:

The original thinking on this (a while ago now) was that contracts can't sign messages (at least this holds true for ethereum), so if the caller is a contract, return the error variant.

As for the nonce, etc, I think this needs to be handled by the specific contract implementation as opposed to in the stdlib.

[adlerjohn]

Ah I see. Well it depends how this is going to be used. Either the signature has to be an Option, or contracts will have two separate methods to either signature- or other-authenticate the sender. If we're going down the route of this PR, then it should probably be the former, and there should be some libraries to handle nonce management.

[nfurfaro]

I left this PR in draft status as I think the topic deserves some discussion around how we want to expose the different auth methods. I was going for the " two separate methods" approach to authentication. The way I've split things up currently, the msg_sender() function will handle items 1 & 3 (I haven't added the get_coins_owner() func yet )in the auth module proposal, and get_signer() would handle item number 2.

Related, ec_recover() and ec_recover_address() need to be refactored to return Results, and then get_signer() can act accordingly depending on the variant it receives from ec_recover_address() This is tracked here: FuelLabs/sway#1067 as well as in a TODO comment in the get_signer() code.

[nfurfaro]

As for failing CI, I think it's because CI installs Forc with cargo, but the latest changes on Sway master haven't been published yet...

[nfurfaro]

Update: I will pick this up again when I wrap up adding tests to the auth module and the reentrancy check functionality for the stdlib.