Support Cosmos SDK secp256k1 signature verification

[ethanfrey]

Part of #751
Requires #780

We start only focusing on the Api trait and basic testing. That means we will do the following in cosmwasm-std crate:

• Add needed methods to Api trait
• Implement them in MockApi importing portable crates
• Implement them in ExternalApi via panic!("not yet implemented")
• Add a test contract with unit tests that call this MockApi to show full stack test (no integration tests yet)
• Add test vectors - directly on MockApi

Add a method that verifies a public key and signature in the format used by the cosmos-sdk (uncompressed).

[webmaster128]

This needs #780 plus contract side pre-hashing (sha256).

[webmaster128]

This is done and demonstrated in https://github.com/CosmWasm/cosmwasm/tree/main/contracts/crypto-verify. Encoding the Cosmos SDK transaction to sign bytes is left to the contract developer.