Reject session proposals on invalid namespace, unsupported methods or no accounts

[orhoj]

Purpose

Reject WalletConnect session proposals that contain invalid namespaces, methods or if there are no accounts in the wallet to create a session for. This is in preparation to support the case where a dApp requests the method for identity proofs, but where the wallet is not a seed phrase wallet, and hence does not have the randomness required to generate proofs. In that scenario we want the wallet to reject the session proposal.

Changes

• Reject session proposal if it does not contain the accepted namespace.
• Reject session proposal if the wallet does not contain any accounts yet (this could be relaxed in the future, if verifiable credentials are added to the wallet).
• Reject session proposal if it requests an unsupported method.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[Radiokot]

👌🏻