Extract mocks protos from the ibc-proto crate into the ibc crate

[romac]

Summary

Extract mocks protos from the ibc-proto crate into the ibc crate.

Problem Definition

@ancazamfir pointed out to me today that the mock proto are kept in sync with our codebase while the rest of the proto definitions might evolve at a different pace. This causes a lot of pain when we want to update the mocks without doing a full release of the ibc-proto crate, or when we want to do an update and release of the ibc-proto crate independently from the rest of the ibc-rs project.

Proposal

Extract the mock protos from the ibc-proto crate into the ibc crate, and compile them to Rust automatically at build time with `build.rs.

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