v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/plugin/client.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"github.com/hashicorp/go-plugin"
+)
+
+const (
+	InternalKeyPrefix = "mmi_"
+	BotUserKey        = InternalKeyPrefix + "botid"
+)
+
+// Starts the serving of a Mattermost plugin over net/rpc. gRPC is not yet supported.
+//
+// Call this when your plugin is ready to start.
+func ClientMain(pluginImplementation any) {
+	if impl, ok := pluginImplementation.(interface {
+		SetAPI(api API)
+		SetDriver(driver Driver)
+	}); !ok {
+		panic("Plugin implementation given must embed plugin.MattermostPlugin")
+	} else {
+		impl.SetAPI(nil)
+		impl.SetDriver(nil)
+	}
+
+	pluginMap := map[string]plugin.Plugin{
+		"hooks": &hooksPlugin{hooks: pluginImplementation},
+	}
+
+	plugin.Serve(&plugin.ServeConfig{
+		HandshakeConfig: handshake,
+		Plugins:         pluginMap,
+	})
+}
+
+type MattermostPlugin struct {
+	// API exposes the plugin api, and becomes available just prior to the OnActive hook.
+	API    API
+	Driver Driver
+}
+
+// SetAPI persists the given API interface to the plugin. It is invoked just prior to the
+// OnActivate hook, exposing the API for use by the plugin.
+func (p *MattermostPlugin) SetAPI(api API) {
+	p.API = api
+}
+
+// SetDriver sets the RPC client implementation to talk with the server.
+func (p *MattermostPlugin) SetDriver(driver Driver) {
+	p.Driver = driver
+}