[cw/light-client] verify membership methods don't have access to CosmWasm context

[rnbguy]

To avoid wasm blob size bloating, we are using cosmwasm's secp256k1 verifier as part of their native environment import. We successfully added this for client update in #205.

Now, similar to #228, we need to validate the commitment proofs too.

But to verify this commitment proof inside the cosmwasm light client, we implement them on ClientStateCommon -- but this trait doesn't pass the ClientValidationContext. So, we can't really use the cosmwasm's native secp256k1 verifier.

Currently, we are considering forking https://github.com/informalsystems/cosmwasm-ibc and adding the signature verifier calls directly at Context implementation.

PS. this issue is redundant if #40 is resolved.