MM-56402: Introduce a pluginID to track RPC DB connections (#26424)

Previously, we relied on the plugin to close the DB connections
on shutdown. While this keeps the code simple, there is no guarantee
that the plugin author will remember to close the DB.

In that case, it's better to track the connections from the server side
and close them in case they weren't closed already. This complicates
the API slightly, but it's a price we need to pay.

https://mattermost.atlassian.net/browse/MM-56402

```release-note
We close any remaining unclosed DB RPC connections
after a plugin shuts down.
```


Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/plugin/driver.go

@@ -62,3 +62,14 @@ type Driver interface {
 	// ResetSession(ctx context.Context) error
 	// IsValid() bool
 }
+
+// AppDriver is an extension of the Driver interface to capture non-RPC APIs.
+type AppDriver interface {
+	Driver
+
+	// ConnWithPluginID is only used by the server, and isn't exposed via the RPC API.
+	ConnWithPluginID(isMaster bool, pluginID string) (string, error)
+	// This is an extra method needed to shutdown connections
+	// after a plugin shuts down.
+	ShutdownConns(pluginID string)
+}