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/helper.go

@@ -0,0 +1,30 @@
+package channeltype
+
+import "github.com/songquanpeng/one-api/relay/apitype"
+
+func ToAPIType(channelType int) int {
+	apiType := apitype.OpenAI
+	switch channelType {
+	case Anthropic:
+		apiType = apitype.Anthropic
+	case Baidu:
+		apiType = apitype.Baidu
+	case PaLM:
+		apiType = apitype.PaLM
+	case Zhipu:
+		apiType = apitype.Zhipu
+	case Ali:
+		apiType = apitype.Ali
+	case Xunfei:
+		apiType = apitype.Xunfei
+	case AIProxyLibrary:
+		apiType = apitype.AIProxyLibrary
+	case Tencent:
+		apiType = apitype.Tencent
+	case Gemini:
+		apiType = apitype.Gemini
+	case Ollama:
+		apiType = apitype.Ollama
+	}
+	return apiType
+}