Add platform api for request validation to reject requests that don't fit warmup shapes

[joerunde]

From talking to @njhill, there's already precedent for doing hardware-specific request validation with the supports_structured_output platform api. For spyre we need a way to reject requests that don't fit a warmup shape, so we should propose a more general api to validate requests up front. Maybe something like:

def validate_request(
        cls,
        prompt: PromptType,
        params: Union[SamplingParams, PoolingParams],
        lora_request: Optional[LoRARequest] = None,
) -> None:
        """Raises if this request is unsupported on this platform"""

[joerunde]

Finally finished in #132