PKCE Verifier in auth code exchange

[karllhughes]

I noticed while setting up FusionAuth with a client-side only application that I don't believe this library supports the PKCE flow outlined here: https://fusionauth.io/learn/expert-advice/oauth/definitive-guide-to-oauth-2#52-code-flow--pkce

I think the exchangeOAuthCodeForAccessToken() method in the FusionAuthClient should have a parameter for the code verifier in order to support this flow.

Let me know if I'm missing something, thanks!