[MM-34919] Use Mattermost-Plugin-ID header to pass ID in inter-plugin requests (#17418)

plugin/context.go

@@ -12,5 +12,5 @@ type Context struct {
 	IpAddress      string
 	AcceptLanguage string
 	UserAgent      string
-	SourcePluginId string
+	SourcePluginId string // Deprecated: Use the "Mattermost-Plugin-ID" HTTP header instead
 }