* feat(utils): Add base64url encoding
RFC 7636 (Proof Key for Code Exchange by OAuth Public Clients) requires to encode the data with base64url encoding.
We don't support decoding as we don't need it for PKCE.
* feat(oauth2): Add PKCE support
* make PKCE code verifier optional, fixup comments
---------
Co-authored-by: Awesome Stickz <awesome@stickz.dev>