[MM-18898] Stringify plugin.Log* parameters (#12700)

- Add stringutils with method that "stringify" object slices
  - "stringify" means convert each object to its string representation
- Move plugin.Log* implementations to client_rpc, use stringify method before calling server method
- Exclude Log* methods from generated RPC methods
- No signature change for plugin.Log* API
- Add test in plugin_api_test to use plugin.Log* methods with RPC

plugin/api.go

@@ -663,7 +663,6 @@ type API interface {
 	// LogDebug writes a log message to the Mattermost server log file.
 	// Appropriate context such as the plugin name will already be added as fields so plugins
 	// do not need to add that info.
-	// keyValuePairs should be primitive go types or other values that can be encoded by encoding/gob
 	//
 	// Minimum server version: 5.2
 	LogDebug(msg string, keyValuePairs ...interface{})
@@ -671,7 +670,6 @@ type API interface {
 	// LogInfo writes a log message to the Mattermost server log file.
 	// Appropriate context such as the plugin name will already be added as fields so plugins
 	// do not need to add that info.
-	// keyValuePairs should be primitive go types or other values that can be encoded by encoding/gob
 	//
 	// Minimum server version: 5.2
 	LogInfo(msg string, keyValuePairs ...interface{})
@@ -679,7 +677,6 @@ type API interface {
 	// LogError writes a log message to the Mattermost server log file.
 	// Appropriate context such as the plugin name will already be added as fields so plugins
 	// do not need to add that info.
-	// keyValuePairs should be primitive go types or other values that can be encoded by encoding/gob
 	//
 	// Minimum server version: 5.2
 	LogError(msg string, keyValuePairs ...interface{})
@@ -687,7 +684,6 @@ type API interface {
 	// LogWarn writes a log message to the Mattermost server log file.
 	// Appropriate context such as the plugin name will already be added as fields so plugins
 	// do not need to add that info.
-	// keyValuePairs should be primitive go types or other values that can be encoded by encoding/gob
 	//
 	// Minimum server version: 5.2
 	LogWarn(msg string, keyValuePairs ...interface{})