Refactor: Consolidate Go packages for standard toolchain compatibility

[wlanmac]

Problem
The previous project structure utilized a Bazel-centric layout for Protobuf generation. While this worked within a Bazel workspace, it created significant friction for standard Go
developers:

1. Package Collisions: Multiple subdirectories (e.g., proto/common_definitions and proto/tpm_enrollz) generated code using the same Go package name (attestz). This caused
   "redeclaration" errors in standard Go builds unless complex aliasing was used.
2. Import Friction: Standard Go tools (IDE autocomplete, go build, go get) could not resolve the types correctly because the directory structure did not align with Go's package
   expectations.
3. Toolchain Locking: Non-Bazel users were forced to manually manage protoc and plugins just to consume the library.

Solution
This PR reorganizes the repository to follow idiomatic Go standards while maintaining 100% compatibility with existing Bazel/Blaze workflows:

• Consolidated Proto Package: All generated Go code from .proto files now resides in the proto/ directory under a single package: github.com/openconfig/attestz/proto/attestz.
• Symmetrical Build Support:
  • Go Users: Pre-generated .pb.go files are checked in, allowing for zero-setup go get usage.
  • Bazel Users: Updated BUILD.bazel rules ensure that Bazel continues to manage dependencies and auto-generate code transparently.
• Improved Documentation: Added a "Go Module Usage" section to the README with clear examples for both proto and business logic imports.

Technical Impact

• Interoperability: These changes are wire-compatible. The underlying protobuf package (openconfig.attestz) remains unchanged, ensuring that new clients can communicate with existing
  servers.
• Source Compatibility: This is a source-breaking change for Go developers. Users will need to update their imports to the unified proto package (e.g., replacing epb and cpb aliases
  with a single apb import).
• Maintenance: Updated regenerate-files.sh to work with the consolidated structure.

[bstoll]

I generally like this change because it aligns closer with how we have organized protobufs in other OpenConfig projects. I think it is easier for Gazelle and other tooling to autogenerate BUILD files when there is a 1:1 mapping of proto packages to a directory.

I don't know if it is reasonable to change, but I personally prefer the directory structure of proto/attestz/* instead of having all the files in the main proto/* directory. This makes the general project layout a bit cleaner when adding new/multiple incompatible protos and maintaining backwards compatibility - imagine introducing a proto/attestz_v2/, etc.

[betuls]

Thanks for this cleanup. I agree with colsolidating the proto packages into /proto/attestz.