Adds a new get_multisig function which generates
a new multisig and returns the public keys,
private keys and all script and address types.
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/14886 | PR14886]]
https://github.com/bitcoin/bitcoin/pull/14886/commits/08a4a0f70f545e114bc35dc76056282cb04bf013
Depends on D6083