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/controller/validator/validation.go

@@ -0,0 +1,38 @@
+package validator
+
+import (
+	"math"
+
+	"github.com/Laisky/errors/v2"
+	"github.com/songquanpeng/one-api/relay/model"
+	"github.com/songquanpeng/one-api/relay/relaymode"
+)
+
+func ValidateTextRequest(textRequest *model.GeneralOpenAIRequest, relayMode int) error {
+	if textRequest.MaxTokens < 0 || textRequest.MaxTokens > math.MaxInt32/2 {
+		return errors.New("max_tokens is invalid")
+	}
+	if textRequest.Model == "" {
+		return errors.New("model is required")
+	}
+	switch relayMode {
+	case relaymode.Completions:
+		if textRequest.Prompt == "" {
+			return errors.New("field prompt is required")
+		}
+	case relaymode.ChatCompletions:
+		if textRequest.Messages == nil || len(textRequest.Messages) == 0 {
+			return errors.New("field messages is required")
+		}
+	case relaymode.Embeddings:
+	case relaymode.Moderations:
+		if textRequest.Input == "" {
+			return errors.New("field input is required")
+		}
+	case relaymode.Edits:
+		if textRequest.Instruction == "" {
+			return errors.New("field instruction is required")
+		}
+	}
+	return nil
+}