Feature Request

Describe your feature request related problem:

BR is currently a CLI program. To some DBAs, this interface is not as straightforward as managing via SQL directly.

Describe the feature you'd like:

We would like to invoke BR directly from TiDB via SQL. This involves several parts:

• pingcap/parser need to recognize the BR-related SQL statements.
• pingcap/tidb need to forward the requests to the BR library.
• pingcap/br (this repository) need to refactor a bit so TiDB can import it.

This issue mainly focuses on the last item.

BR itself imports various packages from TiDB. If such TiDB packages depend on BR again, we get an "import cycle". We need to modify BR to avoid those packages. (The cycle is checked at package level, not module level, so go.mod is hopefully irrelevant.)

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: