explain for connection supports global

## Feature Request

**Is your feature request related to a problem? Please describe:**

The current implementation of explain for connection is that if you view the SQL currently executed on a link or the last SQL execution plan executed, you must log in to the corresponding tidb-server to execute the explain for connection. In most service systems, services or users log in to the tidb-server through load balancing devices. Therefore, the login to the specified tidb-server cannot be ensured, resulting in poor user ease-of-use experience.

**Describe the feature you'd like:**

Log in to any tidb-server in the cluster to view the current SQL execution or last SQL execution plan of all links in the cluster.

**Describe alternatives you've considered:**

**Teachability, Documentation, Adoption, Migration Strategy:**