Export validate function

[thomasjm]

I needed this lower-level function to have a little more control over the OIDC flow. In my case, I wanted to manage the HTTP request myself and do some logging around it.

This validate function is essentially the core part of the requestTokens function, minus the constructing/sending/decoding the request, so I think it's useful to export in its own right.

[krdlab]

Thank you for your PR. Indeed, it would be more convenient to export validate function.