Optimize Objectstore Base64URL encoding bits

[to11mtm]

Proposed change

From #549:

While converting things to have System.Text.Json handle Base64 encoding for us, other opportunities came up.

I think we can also obviate a decent amount of code in ObjectStore
(snip)
Although, not sure if we should keep that in same PR or separate, just for sake of sizing... open to opinions here.
Edit: Actually I may be off on that on some level, Base64URL != Base64... so I'd say keep it separated.

CC:
@galvesribeiro (Thanks for the feedback and inspiration on this issue btw! <3)

Use case

Eliminating intermediary string allocations here (i.e. on SHA checks) would be a perf and memory improvement if we do it well.

This would benefit users of ObjectStore, especially in high load environments.

Contribution

I'd be happy to add it to my backlog? 😅 If someone else wants to give it a go, that's awesome too!