Client to support threhsold < keep group size

[nkuba]

New keep can be requested with threshold < group size.

For the key generation part it is required that all members participate in the process, but for signing it should be possible to generate a valid signature when only the threshold of signers cooperate.

We should update the presence announcement to support the signature generation with not the complete group joining.

This should also include testing if all other parts of the process allow for threshold signing.