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/environment.go

@@ -54,7 +54,7 @@ type Environment struct {
 	logger                           *mlog.Logger
 	metrics                          metricsInterface
 	newAPIImpl                       apiImplCreatorFunc
-	dbDriver                         Driver
+	dbDriver                         AppDriver
 	pluginDir                        string
 	webappPluginDir                  string
 	prepackagedPlugins               []*PrepackagedPlugin
@@ -64,7 +64,7 @@ type Environment struct {
 
 func NewEnvironment(
 	newAPIImpl apiImplCreatorFunc,
-	dbDriver Driver,
+	dbDriver AppDriver,
 	pluginDir string,
 	webappPluginDir string,
 	logger *mlog.Logger,