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/adaptor/aiproxy/model.go

@@ -0,0 +1,32 @@
+package aiproxy
+
+type LibraryRequest struct {
+	Model     string `json:"model"`
+	Query     string `json:"query"`
+	LibraryId string `json:"libraryId"`
+	Stream    bool   `json:"stream"`
+}
+
+type LibraryError struct {
+	ErrCode int    `json:"errCode"`
+	Message string `json:"message"`
+}
+
+type LibraryDocument struct {
+	Title string `json:"title"`
+	URL   string `json:"url"`
+}
+
+type LibraryResponse struct {
+	Success   bool              `json:"success"`
+	Answer    string            `json:"answer"`
+	Documents []LibraryDocument `json:"documents"`
+	LibraryError
+}
+
+type LibraryStreamResponse struct {
+	Content   string            `json:"content"`
+	Finish    bool              `json:"finish"`
+	Model     string            `json:"model"`
+	Documents []LibraryDocument `json:"documents"`
+}