Include APK commitment to validator key set to beefy "witness"

[drskalman]

Beefy's commitment to validator set's ECDSA keys is in form of merkle root of a tree containig all ECDSA keys. For the BLS aggregated key to be verifiale by APK proofs we need a KZG commitment to the validator set instead.

[drskalman]

blocked by paritytech/apk-proofs#42

[drskalman]

This is mostly done except that type info is not correctly implement.