Allow len(signatures) == 0 in aggregate function and len(pubkeys) == 0 in verify functions

[hwwhww]

Based on #2068

Also suggested by @JustinDrake, we can go farther to allow empty aggregate signatures in the verification for phases 1+.

Issue

IETF BLS draft 2 requires (preconditions):

1. len(signatures) >= 1 in Aggregate(signatures)
2. len(pubkeys) >= 1 in -AggregateVerify functions.

For phase 1 light_client_signature and shard block proposer_signature_aggregate use cases, there are two phase 1 helper functions were added in #1812 to allow no-signatures case (len(signatures) == 0/len(pubkeys) == 0):

1. optional_aggregate_verify
2. optional_fast_aggregate_verify

Proposed solution

Some BLSv4 compatible wrappers are added in #2068.

If we implement wrappers to allow len(signatures) == 0 in Aggregate(signatures), we could also make -verify function wrappers allow len(pubkeys) == 0 and remove the optional_ helpers.

Note that it's a non-substantive change for phase 0.

[mratsim]

BLS draft 2 requires n >= 1 so empty public keys sets are not allowed: https://tools.ietf.org/html/draft-irtf-cfrg-bls-signature-02#section-3.1.1

Similarly, aggregate requires >= 1 signatures: https://tools.ietf.org/html/draft-irtf-cfrg-bls-signature-02#section-2.8

See also our audit report where we allowed this by missing a check on the precondition: status-im/nim-blscurve#77

[hwwhww]

Died with #2068 😄