Policy struct protobuf definitions

[puerco]

Summary of Changes

This PR introduces a protocol buffers definition for the source policy file, updates the code to use it and includes the supporting tools to manage them. Summary of the changes:

1. The proto files
2. Generated policy.pb.go file
3. Complete update to the codebase to use the new generated types
4. Updated all tests
5. Code generation scripts and CI presubmit check to keep definitions up to date (using buf to manage)

⚠️ Breaking changes

There are two important changes in this PR which are breaking changes but should not have an impact:

• As discussed, the protobuf defintion normalizes the casing on the policy fields, namely: All Since date fields are now since and Name is now name.
• The policy repository constants are now pointing to slsa-framework/source-policies. The new repo already has the existing policies updated to protobuf format. This should not disrupt existing operations as the workflow is pinned, once we bump the sourcetool version it will start using the new constants and read the new policies.

Up Next:

Following up to this PR I'll submit the source provenance predicate proto definitions and updates