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

web/README.md

@@ -2,6 +2,9 @@
 
 > 每个文件夹代表一个主题，欢迎提交你的主题
 
+> [!WARNING]
+> 不是每一个主题都及时同步了所有功能，由于精力有限，优先更新默认主题，其他主题欢迎 & 期待 PR
+
 ## 提交新的主题
 
 > 欢迎在页面底部保留你和 One API 的版权信息以及指向链接