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/baidu/model.go

@@ -0,0 +1,50 @@
+package baidu
+
+// import (
+// 	"github.com/songquanpeng/one-api/relay/channel/openai"
+// 	"time"
+// )
+
+// type ChatResponse struct {
+// 	Id               string       `json:"id"`
+// 	Object           string       `json:"object"`
+// 	Created          int64        `json:"created"`
+// 	Result           string       `json:"result"`
+// 	IsTruncated      bool         `json:"is_truncated"`
+// 	NeedClearHistory bool         `json:"need_clear_history"`
+// 	Usage            openai.Usage `json:"usage"`
+// 	Error
+// }
+
+// type ChatStreamResponse struct {
+// 	ChatResponse
+// 	SentenceId int  `json:"sentence_id"`
+// 	IsEnd      bool `json:"is_end"`
+// }
+
+// type EmbeddingRequest struct {
+// 	Input []string `json:"input"`
+// }
+
+// type EmbeddingData struct {
+// 	Object    string    `json:"object"`
+// 	Embedding []float64 `json:"embedding"`
+// 	Index     int       `json:"index"`
+// }
+
+// type EmbeddingResponse struct {
+// 	Id      string          `json:"id"`
+// 	Object  string          `json:"object"`
+// 	Created int64           `json:"created"`
+// 	Data    []EmbeddingData `json:"data"`
+// 	Usage   openai.Usage    `json:"usage"`
+// 	Error
+// }
+
+// type AccessToken struct {
+// 	AccessToken      string    `json:"access_token"`
+// 	Error            string    `json:"error,omitempty"`
+// 	ErrorDescription string    `json:"error_description,omitempty"`
+// 	ExpiresIn        int64     `json:"expires_in,omitempty"`
+// 	ExpiresAt        time.Time `json:"-"`
+// }