Codec Module

[jmank88]

This PR creates a separate, top-level module codec, which contains the relayer/codec and relayer/chainreader/loop packages. This new module has ~100 less dependencies than the main module. The old packages are deprecated but remain compatible by forwarding to the new module.

Supports:

• use chainlink-aptos/codec module chainlink#23011

TODO:

• Does CI run the unit tests in their new location?
• Split into two PRs to satisfy CI check for non-default branch commit hashes