Add OpenAIEmbeddings special token params for tiktoken

[shibanovp]

#2681

Original type hints

allowed_special: Union[Literal["all"], AbstractSet[str]] = set(),  # noqa: B006
disallowed_special: Union[Literal["all"], Collection[str]] = "all",

from
https://github.com/openai/tiktoken/blob/46287bfa493f8ccca4d927386d7ea9cc20487525/tiktoken/core.py#L79-L80 are not compatible with pydantic

I think we could use

allowed_special: Union[Literal["all"], Set[str]] = set()
disallowed_special: Union[Literal["all"], Set[str], Tuple[()]] = "all"

Please let me know if you would like to implement it differently.

[vowelparrot]

This looks nice - thanks