plugin: Allow introduce a custom web handler

Feature Request

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

TiDB contains an HTTP interface at port :10080. A plugin may want to expose some status and configuration interface through this port.

Describe the feature you'd like:

Allow the plugin to provide an http.Handler from the Manifest.

type Manifest struct {
    ...
    Handler  http.Handler
}

Inside startHTTPServer(), for every plugin which has a non-nil Handler, route /plugins/«Name»/ to that handler (with StripPrefix).

Describe alternatives you've considered:

Do not support it, forcing plugins wanting to provide a web interface to choose a different port.

Teachability, Documentation, Adoption, Migration Strategy: