Add support for signed payloads to the `setOptions` operation.

[Shaptic]

While many parts of the SDK have been updated to enable the new signer type from CAP-40, the SetOptions operation has not.

As part of this, new helpers exist:

  function composeSignedPayload(address: string, payload: Buffer): string;
  function decomposeSignedPayload(address: string): { signer: string; payload: Buffer };

which should make it easy to create a P-address from a G-address and a payload and vice-versa.