CSUB-518: Pt. 2 - Set Gate Faucet Address

[zacharyfrederick]

Rationale

A successful GCRE->CTC swap results in a minting of CTC.

A successful swap of GATE results in a transfer, where the source of funds is an on-chain storage object called the gate faucet address, and the destination is the signer of the extrinsic.

This gate faucet must be set before the request_collect_coins_v2 extrinsic is called. If the on-chain storage value has not been set an error will be raised.

This PR adds a set_gate_faucet extrinsic that allows a key with sudo privileges to specify which on-chain wallet is to be used as the source argument in the transfer function call.

The value gets persisted as an OptionQuery for an AccountId. An OptionQuery will return the None variant when not set.

The getter for the faucet address is Creditcoin::gate_faucet_address. Its definition is the following:

pub type GATEFaucetAddress<T: Config> = StorageValue<_, T::AccountId, OptionQuery>;

Unit tests implemented:
- [x] When not previously set storage value should return the correct option (None)
- [x] The extrinsic can only be set by a root user
- [x] The extrinsic can be set and the storage value is persisted
- [ ] add TODO's here

This PR is based off of Pt. 1. When Pt 1 gets merged into dev this part will get rebased. Will likely need to fix merge conflicts caused by creditcoin-js updates. This pattern will continue for part 3 and 4.

This extrinsic can be called on the javascript side like so:

 await api.tx.sudo
     .sudo(api.tx.creditcoin.setGateFaucet(address))
     .signAndSend(sudoSigner, { nonce: -1 });

[codecov-commenter]

Codecov Report

Merging #1276 (5814f81) into dev (863fad1) will not change coverage.
The diff coverage is n/a.

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  Coverage   8.32%   8.32%           
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  Files         28      28           
  Lines        889     889           
  Branches     114     114           
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  Hits          74      74           
  Misses       815     815           

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[github-actions]

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[atodorov]

Working on an integration test for set_gate_contract() and will merge that one first, before merging this one.

Otherwise looks good.

[nathanwhit]

A couple of nitpicks but the actual content LGTM