IF: Generate BLS key pairs in leap-util

[arhag]

Depends #1584 and #1596.

Ensure changes in AntelopeIO/bls12-381#10 are brought into Leap as a submodule update.

There is no need for cleos to create a BLS key pair since BLS key pairs are only needed by some node operators (block producer candidates).

But the basic interface for creating key pairs for other key types (K1, R1) in cleos should be adopted in leap-util but specifically for creating a BLS key pair (there is no need for leap-util to generate K1 or R1 keys).

However, it should also generate a Proof of Possession (PoP) and include that signature in the output as well to act as a proof that the private key for the public key is known. This will be necessary for registering the public key as a finalizer key later in the system contracts. Use https://github.com/AntelopeIO/bls12-381/blob/d79a1836649d6f9b7494ccc3e9a1320a7470acaa/src/signatures.cpp#L630 to generate the PoP.

There should also be another sub-command to regenerate the PoP from a given private key. This will not only output the signature but also the public key corresponding to the provided private key.

[linh2931]

Resolved by #1594

[arhag]

Reopening because #1594 does not use pop_prove but should.