MM-10702 Moving plugins to use hashicorp go-plugin. (#8978)

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

plugin/hooks.go

@@ -9,15 +9,32 @@ import (
 	"github.com/mattermost/mattermost-server/model"
 )
 
+// These assignments are part of the wire protocol. You can add more, but should not change existing
+// assignments. Follow the naming convention of <HookName>Id as the autogenerated glue code depends on that.
+const (
+	OnActivateId            = 0
+	OnDeactivateId          = 1
+	ServeHTTPId             = 2
+	OnConfigurationChangeId = 3
+	ExecuteCommandId        = 4
+	MessageWillBePostedId   = 5
+	MessageWillBeUpdatedId  = 6
+	MessageHasBeenPostedId  = 7
+	MessageHasBeenUpdatedId = 8
+	TotalHooksId            = iota
+)
+
 // Methods from the Hooks interface can be used by a plugin to respond to events. Methods are likely
 // to be added over time, and plugins are not expected to implement all of them. Instead, plugins
 // are expected to implement a subset of them and pass an instance to plugin/rpcplugin.Main, which
 // will take over execution of the process and add default behaviors for missing hooks.
 type Hooks interface {
-	// OnActivate is invoked when the plugin is activated. Implementations will usually want to save
-	// the api argument for later use. Loading configuration for the first time is also a commonly
-	// done here.
-	OnActivate(API) error
+	// OnActivate is invoked when the plugin is activated.
+	OnActivate() error
+
+	// Implemented returns a list of hooks that are implmented by the plugin.
+	// Plugins do not need to provide an implementation. Any given will be ignored.
+	Implemented() ([]string, error)
 
 	// OnDeactivate is invoked when the plugin is deactivated. This is the plugin's last chance to
 	// use the API, and the plugin will be terminated shortly after this invocation.
@@ -31,7 +48,7 @@ type Hooks interface {
 	//
 	// The Mattermost-User-Id header will be present if (and only if) the request is by an
 	// authenticated user.
-	ServeHTTP(http.ResponseWriter, *http.Request)
+	ServeHTTP(w http.ResponseWriter, r *http.Request)
 
 	// ExecuteCommand executes a command that has been previously registered via the RegisterCommand
 	// API.