Support multiple drainers

[sokada1221]

Feature Request

Is your feature request related to a problem? Please describe:
Currently you can only deploy one drainer per TiDB cluster.

Describe the feature you'd like:
Ability to deploy multiple drainers per TiDB cluster.

Describe alternatives you've considered:
Chaining multiple TiDB clusters would work, but it's not ideal - increased dependency/complexity and potential lag problem.

Teachability, Documentation, Adoption, Migration Strategy:
Example scenario would be:

1. Use one drainer to replicate to another TiDB cluster or MySQL; and
2. Use another drainer to dump incremental backup

Just like how the architecture diagram shows here:
https://pingcap.com/docs/v3.0/reference/tidb-binlog-overview/?origin_team=T0351U5DY#tidb-binlog-cluster-overview