- Creates the license declaration.
- Applies it to src:*
- Add tools/compliance to demonstrate how we will gather all the license
  - bazel build //tools/compliance:*
  - cat bazel-bin/tools/compliance/*.txt

Closes #15543.

PiperOrigin-RevId: 450909439
Change-Id: Id3b249592ade8089583edb14bf91af930810f8f2