Exposing HTTP interface of current task status

Feature Request

Is your feature request related to a problem? Please describe: Currently, some components (like BR) are incompatible with TiCDC, when running them at clusters with TiCDC, we will meet some fetal problems: including TiCDC crash, inconsistency of upstream and downstream, etc..

So we need to deny executing those components when TiCDC presents. But currently, the only way to test whether TiCDC exists at cluster is to query the embedded Etcd of PD. Which introduces coupling of the keys in Etcd and other components.

Describe the feature you'd like:

Add a HTTP interface to expose some task information. Maybe including:

• The current running task count.
• The tables/DBs that TiCDC is pulling.

Describe alternatives you've considered:

In another way, since we just to test whether TiCDC presents, current Etcd keys is in some way enough, some times even better (HTTP interface doesn't work well when TiCDC use non-standard ports). But we should make sure that the keys in Etcd we used (just a subset of keys TiCDC uses) shouldn't be changed.

Teachability, Documentation, Adoption, Migration Strategy:

TODO.