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/channeltype/define.go

@@ -0,0 +1,39 @@
+package channeltype
+
+const (
+	Unknown = iota
+	OpenAI
+	API2D
+	Azure
+	CloseAI
+	OpenAISB
+	OpenAIMax
+	OhMyGPT
+	Custom
+	Ails
+	AIProxy
+	PaLM
+	API2GPT
+	AIGC2D
+	Anthropic
+	Baidu
+	Zhipu
+	Ali
+	Xunfei
+	AI360
+	OpenRouter
+	AIProxyLibrary
+	FastGPT
+	Tencent
+	Gemini
+	Moonshot
+	Baichuan
+	Minimax
+	Mistral
+	Groq
+	Ollama
+	LingYiWanWu
+	StepFun
+
+	Dummy
+)