Enable generating multiple FROST keys

[mpetrun5]

As we can't add peers to the existing FROST keys, we want to be able to generate multiple FROST keys
and configure by address which key is used to sign the message.

Implementation details

• change the FROST key path to a folder
• allow for generating mutiple keys and store them by their public key
• set in the configuration the MPC public key per BTC deposit address

Testing details

• unit tests
• e2e tests

Acceptance Criteria

• we are able to have multiple FROST keys