SecretStore: threshold ECDSA PoC (#7615)

* SecretStore: ECDSA PoC

* SecretStore: fixed ECDSA serialization + cleanup

* removed unused param

* removed unused method

* removed debug unwrap

* 1/x -> inv(x)

* SecretStore: merged fixes from ECDSA session branch

* once again  1/* -> inv(*)

* fixed grumbles