Add base64url support

[olehmisar]

Problem

I want to encode/decode a base64url string

Happy Case

I can encode/decode a base64url string

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

Nice-to-have

Blocker Context

No response

Would you like to submit a PR for this Issue?

Maybe

Support Needs

No response

[saleel]

I believe @grjte is working on adding this.

[grjte]

Yep, I'll open a PR soon! Just doing some profiling

[grjte]

@saleel I have branches ready to 1) add support for optional padding and 2) add base64url support, which is a very small follow-up once the optional padding is done. They both work off of the formatting fix PR I opened last week #18, so I can open a PR once that one (or a similar fix) is merged.

I do have one point of confusion where I could really use a sanity check - I ran bb gates over encode/decode with the current version (on main), the version with optional padding and the version with base64url. According to my results, the costs don't change between them (which is what I expect, if globals are only executed when they're used rather than when they're declared), but it also shows a different result for the current main version than when I tested to update the README previously, so I made a profiling error at some point. It'd be really helpful to get feedback here if you have a chance to look.