Allow signing and showing addresses without registering policies, or without policies at all

[bigspider]

Currently, for any wallet policy which is not one of the standard wallet policies, it is necessary to register it first, and then use the registration hmac in calls to get_wallet_address or sign_psbt.

It might be useful to allow unregistered (and unnamed) wallet policies, where the policy verification (similarly to the registration flow) is executed right before showing the address or signing a transaction, and no hmac is returned.
This is particularly useful since policies are not revocable, forcing registration when a one-time usage is desired could by itself be a security risk.

[bigspider]

As per the discussion in #153, there are some use cases that don't quite fit the "wallet policy" model (at least in the current version), yet it would be useful to be able to sign with keys controlled by the hardware wallet.

Without wallet policies, it might still be considered safe to sign with the internal keys if (for example) there is no change address and the user inspects all the outputs.

One concern mentioned in the discussion is that it would not be easy (or at all possible) to make sure that the UTXO being signed for does not belong to any of the registered wallet policies. As long as the UX makes it abundantly clear that it's not signing from a wallet policy, this might be acceptable. The policy-less signing could perhaps be disabled by default and only enabled with an explicit setting of the app.