Alter the BEEFY structure to have only one aggergated BLS signature

[drskalman]

According to https://github.com/paritytech/substrate/pull/10466#issuecomment-1144648900 BLS signatures because they can be aggregated, they should be treated differently rather than put in the vector of validator signatures. This issue intends to add an extra field as aggrigatable signature generic type to BEEFY primitve type.

[drskalman]

test for bls signature contating commitment added: 577cc68a1d8