Refactor codebase, introduce relaymode package, update constants and improve consistency

- Refactor constant definitions and organization
- Clean up package level variables and functions
- Introduce new `relaymode` and `apitype` packages for constant definitions
- Refactor and simplify code in several packages including `openai`, `relay/channel/baidu`, `relay/util`, `relay/controller`, `relay/channeltype`
- Add helper functions in `relay/channeltype` package to convert channel type constants to corresponding API type constants
- Remove deprecated functions such as `ResponseText2Usage` from `relay/channel/openai/helper.go`
- Modify code in `relay/util/validation.go` and related files to use new `validator.ValidateTextRequest` function
- Rename `util` package to `relaymode` and update related imports in several packages

relay/relaymode/define.go

@@ -0,0 +1,14 @@
+package relaymode
+
+const (
+	Unknown = iota
+	ChatCompletions
+	Completions
+	Embeddings
+	Moderations
+	ImagesGenerations
+	Edits
+	AudioSpeech
+	AudioTranscription
+	AudioTranslation
+)