fix(fc-pallet-pass): prevent reusing credentials

[pandres95]

This PR includes some tests that are failing because it's possible to reuse some credentials that have been found out there to impersonate a pass account. This is completely inadmissible, as it poses a huge security risk, and must be fixed immediately. This PR fixes it.

Superseeds #40, as this solution uses the Transaction Extensions approach.

How the fix works

1. Challenger::generate and Challenger::validate_challenge now include a new parameter: extrinsic_context: impl AsRef<[u8]>. This parameter should be processed by the challenger.
2. Pass::authenticate includes an additional parameter: extrinsic_context: impl AsRef<[u8]>.
3. We pass the encoded value of inhereted_implication from TransactionExtensions::validate to Pass::authenticate.

Since this value changes on each new transaction (because the nonce changes) we ensure each

What to change in external authenticators

External authenticators now must include the external challenge using this calculation¹:

1. The extension version byte, call, extension and extension implicit should be encoded (by "extension" and its implicit we mean only the data associated with extensions that follow this one in the composite extension type);
2. The result of the encoding should then be hashed using the BLAKE2_256 hasher;

// Step 1: encode the bytes
let encoded = (extension_version_byte, call, transaction_extensions, transaction_extensions_implicit_values).encode();
// Step 2: hash them
blake2_256(&encoded[..])

Footnotes

1. As defined by RFC 124 from the Polkadot Fellowship. ↩