[Feature]: Add supporting for backing-up and restoring encrypted DKG shares

[aldur]

Feature - Add supporting for backing-up and restoring encrypted DKG shares

1. Description

DKG shares do not currently depend on the private key specified in the config, but are generated through a PRNG.

They are stored, encrypted, in the DB.

A signer that loses the DB would mean losing a share of the corresponding aggregate key.

1.1 Context & Purpose

Long-term, we should consider deterministically deriving DKG shares based on some recoverable seed. See #1303.

Short-term, we should make it easy for sBTC signers to:

1. Export their encrypted DKG shares.
2. Restore their encrypted DKG shares.

This would make backup operations easier.

2. Technical Details:

2.1 Acceptance Criteria:

• As an sBTC signer, I can run the sBTC binary with a CLI flag that allows me to export the encrypted DKG shares.
• As an sBTC signer, I can run the sBTC binary with a CLI flag that allows me to restore the encrypted DKG shares (adding to existing ones or failing if the DB is already populated).