add max_context_size property in BaseOpenAI

[ikebo]

Hi, I make a small improvement for BaseOpenAI.

I added a max_context_size attribute to BaseOpenAI so that we can get the max context size directly instead of only getting the maximum token size of the prompt through the max_tokens_for_prompt method.

Who can review? @hwchase17 @agola11

I followed the Common Tasks, the test is all passed.