feat: add optional signEip7702Authorization method to Keyring type (#231)

EIP-7702 defines a new struct Authorization which represents authority
to set a pointer to a contract address at an EOA - effectively making
the EOA perform as a smart contract.

This change adds an optional `signEip7702Authorization` method to the
`Keyring` type which is implemented on `@metamask/eth-simple-keyring`
and `@metamask/eth-hd-keyring` in
MetaMask/accounts#182.

See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-7702.md for
details

---------

Co-authored-by: Charly Chevalier <charly.chevalier@consensys.net>

Author: jeffsmale90

src/keyring.ts

@@ -169,6 +169,22 @@ export type Keyring<State extends Json> = {
     options?: Record<string, unknown>,
   ): Promise<string>;
 
+  /**
+   * Sign an EIP-7702 authorization. This is a signing method for authorizing a
+   * specific contract on a specific chain.
+   *
+   * @param address - The address of the account to use for signing.
+   * @param authorization - An array containing the chain ID, contract address,
+   * and nonce.
+   * @param options - Signing options; differs between keyrings.
+   * @returns The signed authorization as a hex string.
+   */
+  signEip7702Authorization?(
+    address: Hex,
+    authorization: [chainId: number, contractAddress: Hex, nonce: number],
+    options?: Record<string, unknown>,
+  ): Promise<string>;
+
   /**
    * Sign a message. This is equivalent to the `eth_sign` Ethereum JSON-RPC
    * method, which is exposed by MetaMask as the method `personal_sign`. See