Same feedback as https://phab.comm.dev/D7649#inline-50356

TLDR: I think we need to think about this as replacing base64 encoding with base64url encoding. Regex approach may be fine, but we should also handle replacing + with -. While the + might not be causing any issues at the moment, I think it would be a better approach than having a custom base64/base64url encoding that happens to work.

We don't usually use snake case on the JS side. I'm guessing this corresponds to something on the Rust side where snake case might be the usual convention?

Probably fine, but I think we can "automate" this via some sort of Rust Serde macro(?). Doesn't look like we use this in our codebase, but saw it used in another one I was looking at recently:

#[serde(rename_all = "snake_case")]

But maybe this is too "magical," defer to {O4}