[GSoC26][bazel] add bazel build system support for simple_switch_grpc

[c8ef]

Introduces a complete bazel build alongside the existing cmake setup, targeting the simple_switch_grpc binary and its test suite.

Key implementation notes:

• All BUILD files are formatted with Buildifier and enforced in CI via a dedicated GitHub Actions workflow, keeping the Starlark codebase consistent from day one.
• config.h is generated using the expand_template idiom rather than a raw genrule, making the enabled/disabled feature set explicit and diff-friendly.
• External dependencies (Boost, gmp, nanomsg, grpc, p4lang/PI, etc.) are currently consumed as system-installed libraries through a module extension (bazel/ubuntu2204.bzl). This avoids the need to vendor or pin every transitive dependency upfront; migration to BCR-sourced modules can be done incrementally.
• Groundwork for a full Bazel-native p4lang/PI is already underway: uthash@2.3.0 bazelbuild/bazel-central-registry#8031 tracks a third-party library used by the PI library's contribution to BCR, after which the @ubuntu2204//:pi* stubs can be replaced with proper module dependencies.

[c8ef]

cc @matthewtlam

[matthewtlam]

Thanks for your PR @c8ef.

None of the PRs will be carefully reviewed until Apr 1 (once the contributor deadline is over). The GSoC contributor application is currently opened. Please make sure you submit your application before the deadline.

During the first week of April, after applications and PRs are reviewed, candidates who will be considered will be sent an interview. The contributor who gets admitted into GSoC will have their PR pushed to head

cc: @qobilidop