- 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
- Introduced idle connection timeout for HTTP client transport
- Implemented custom timeout for HTTP client when `RelayTimeout` is 0
- Set ImpatientHTTPClient timeout to 5 seconds
- Added new config variable `IdleTimeout` with default value of 30 seconds
- Utilized shared transport object for all HTTP clients in relay/util package
- Update the minimum access token length from 16 to 32
- Prevent spam by introducing policies and detecting user agents
- Add an authorization header to the login response
- Use base64 to decode the session secret and generate a random one if not set
