Modify the signing by Fireblocks provided `keyId`

Allows assigning the signing key to a vault account, if it hasn't been assigned to any other vault accounts yet.
Please note that this endpoint is available only for Key Link enabled workspaces.
This endpoint is currently in beta and might be subject to changes.
If you want to participate and learn more about the Fireblocks Key Link, please contact your Fireblocks Customer Success Manager or send an email to [email protected].