update ext_crypto_start_batch_verify and ext_crypto_finish_batch_verify to use go-schnorrkel BatchVerify

[noot]

Task summary

• see https://github.com/ChainSafe/go-schnorrkel/blob/master/batch.go#L69
• now, when starting batch, we can create a new schnorrkel.BatchVerifier
• use Add to add a new signature, and Verify to finish verification

[noot]

can use this library for ed25519 batch verification: https://github.com/hdevalence/ed25519consensus/blob/main/batch.go

[danforbes]

@noot I have removed this from the cross-client devnet Epic for now #1356 - does that seem correct? I think you mentioned that these methods may be deprecated, although they still seem to appear in the spec (ref https://w3f.github.io/polkadot-spec/develop/sect-crypto-api.html#sect-ext-crypto-start-batch-verify). I think this may still be worth doing but I don't see how it directly relates to #1356

[timwu20]

This isn't required anymore. They've deprecated these host api functions.