…icies to more general key expressions

…ed some comments.

Generalizing to key expressions containing musig() makes it necessary to distinguish
the key expressions in the wallet policy from the actual key placeholders that are
just indexes to the list of key informations (@num in the descriptor template),
whereas the two concepts were often not clearly separated in the code base.

Renaming to "key expressions" makes the distinction more clear.

…on type is used; generalized some parts of the code that are not generalized to musig key expressions, and annotated some others.

…t library

…ues in sign_psbt in the python client

…ting prematurely for some PSBTs. Added explanatory comment

The check doesn't serve a security purpose, since direct derivation of each input/change script
is used to check if they are internal.

…ey_placeholders, with musig placeholder support

…support

…l the internal key expressions, instead of only the first

…ies with multiple internal keys

It will be needed in order to enable silently participating to Round 1
of the MuSig2 protocol, which should only happen if no such fields are
found (which would indicate that Round 1 was already executed).

…onfirmation.

The first round of MuSig does not involve access to the private keys and does not
complete authorizing a transaction. Therefore, it is safe to do it without user
confirmation, which allows software wallets to possibly do it in background.

In cases when the other cosigners are online, this allows to get a single-sig
user experience, as the user would only have to plug the device once, and confirm
a single action.

…ded_pubkey_from_client