Add support for BYOT (Bring Your Own Type) in the /v1/chat/completions endpoint, allowing clients to pass custom type definitions and schemas in requests.

The completions API currently uses predefined types (ChatCompletionRequest, ChatCompletionRequestWithTools) with fixed schemas for tools, functions, and response formats. Some use cases require custom type definitions that aren't covered by the standard types.

Users cannot:

• Define custom tool/function schemas beyond the standard JSON Schema format
• Pass custom type definitions for structured outputs
• Extend the request/response types with domain-specific schemas
• Use custom validation rules or type constraints
• This limits flexibility for advanced use cases that need domain-specific type systems or custom validation.