[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

app/plugin_api_test/plugin_using_log_api.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2019-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package main
+
+import (
+	"github.com/mattermost/mattermost-server/plugin"
+	"github.com/pkg/errors"
+)
+
+type PluginUsingLogAPI struct {
+	plugin.MattermostPlugin
+}
+
+type Foo struct {
+	bar float64
+}
+
+func main() {
+	plugin.ClientMain(&PluginUsingLogAPI{})
+}
+
+func (p *PluginUsingLogAPI) OnActivate() error {
+	p.API.LogDebug("LogDebug", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogInfo("LogInfo", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogWarn("LogWarn", "one", 1, "two", "two", "foo", Foo{bar: 3.1416})
+	p.API.LogError("LogError", "error", errors.WithStack(errors.New("boom!")))
+	return nil
+}