feat: base64-encode session cookie value

[jebibot]

What kind of change does this PR introduce?

Feature

What is the current behavior?

The session cookie value is a plain JSON string, which gets URL-encoded by most server implementations. This causes a mismatch in the length calculation in the chunker; sometimes, cookie values exceed the max length.

What is the new behavior?

This PR base64-encodes the cookie value before passing to the chunker.

The rationale for using base64:

• it is used in the JWT encoding
• jose has been included and used for while
• URL encoding encodes {, }, ", ] to 3 characters tripling in length, whereas base64 encoding increases x4/3 in bytes, so the overall length would be similar

Additional context

[jebibot]

Closing in favor of supabase/auth-js#816, as auth-helpers are deprecated.