Support OCSP stapling

[colmmacc]

s2n should support OCSP stapling. Implementing HTTP and calling an OSCP endpoint is likely overkill, but it should at least be possible for the caller to periodically provide an OCSP token.

[baldwinmatt]

I have request/response implemented.

next up is validating the status

[baldwinmatt]

This requires some thought. As a client, we should give the application notice that OCSP validation failed, and allow the user/programmer to make a decision on what to do next.

For now, I'll implement an API to retrieve the OCSP response.