4 governed contracts for whitelisting cw721 contracts and ICS721 channels.

[taitruong]

4 governed proxy implementations for ICS721 usage:

1. cw721-code-id-proxy
2. cw721-sender-proxy
3. ics721-channel-proxy
4. ics721-sender-channels-proxy

These can then be used by ICS721 here:
https://github.com/public-awesome/ics721/blob/main/contracts/cw-ics721-bridge/src/msg.rs#L18-L22
https://github.com/public-awesome/ics721/blob/main/contracts/cw-ics721-bridge/src/contract.rs#L431

[taitruong]

PR contains now 2 contracts: contracts/cw721-sender-whitelist-proxy and contracts/ics721-channel-whitelist-proxy. 2 thoughts on this:

1. both contracts are from a functional point of view. maybe there is a better approach of merging both into a more generic wl proxy.
2. another contract icw721-sender-and-channel-whitelist-proxy would be great. Also here: maybe there is a better (rule-based?) solution than just doing copy-pasta.