Add better support for building and testing through Bazel.

[davidlion]

Description

Previously building and testing through bazel was only partially supported and required non-trivial additions into the module using the library.

This PR adds support to completely build and test the library directly though bazel.
The main addition is adding a bazel module extension to support building the FFI core as a library from the CLP submodule. This allows bazel projects depending on ffi-go to pull it in with minimal work.

Validation performed

Build and testing though bazel added to workflows.
Imported the library into another bazel project locally.

[junhaoliao]

LGTM. There were some minor changes I had to make to get this integrated into a monorepo which has not yet migrated to Bzlmod. Let's discuss offline to see if any process can be optimized.

[junhaoliao]

Do we need a WORKSPACE? (Would it help projects which haven't migrated to Bzlmod?)

[junhaoliao]

REPO.bazel does seem good to have for specifying the OSS license for all sources: https://bazel.build/external/overview#repo.bazel Do we want to add such?

[davidlion]

Do we need a WORKSPACE? (Would it help projects which haven't migrated to Bzlmod?)

Afaik, this would only enable building ffi-go itself through old versions of bazel, which isn't necessary to support (on its own ffi-go doesn't really do anything).

REPO.bazel does seem good to have for specifying the OSS license for all sources: https://bazel.build/external/overview#repo.bazel Do we want to add such?

As discussed, exploring https://bazelbuild.github.io/rules_license/latest.html

[junhaoliao]

I was able to integrate the latest commit into a WORKSPACE (non-Bzlmod) project without any modification.

Here is the WORKSPACE configuration:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

def com_github_y_scope_clp_ffi_go():
    _com_github_y_scope_clp_ffi_go_commit = "0d8c24777af639a65a317954eec75554e6b121b0"
    http_archive(
        name = "com_github_y_scope_clp_ffi_go",
        sha256 = "19d2e46f13233a538b2dae103c07f46623ee19e48b5d4bc05348776b8e769d01",
        urls = ["https://github.com/davidlion/clp-ffi-go/archive/{}.zip".format(_com_github_y_scope_clp_ffi_go_commit)],
        strip_prefix = "clp-ffi-go-{}".format(_com_github_y_scope_clp_ffi_go_commit),
    )

...

load("@com_github_y_scope_clp_ffi_go//cpp:deps.bzl", "com_github_y_scope_clp")
com_github_y_scope_clp()

The changes look good to me, and the PR is fine for the squashed commit message.

[kirkrodrigues]

Minor comments. I trust Junhao's review.

[kirkrodrigues]

Shall we group and alphabetize? E.g., something like

# Bazel-generated files
bazel-bin
...

# C++ build generated files
cpp/.cache/
**/compile_commands.json

# Submodules
cpp/clp