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

common/helper/time.go

@@ -0,0 +1,15 @@
+package helper
+
+import (
+	"fmt"
+	"time"
+)
+
+func GetTimestamp() int64 {
+	return time.Now().Unix()
+}
+
+func GetTimeString() string {
+	now := time.Now()
+	return fmt.Sprintf("%s%d", now.Format("20060102150405"), now.UnixNano()%1e9)
+}