Convert to base64url

base64url: the variant JWTs use

+ and / mean something in URLs, and = means something in query strings. So JWTs (and many web formats) use base64url:

• + becomes -, and / becomes _
• trailing = padding is removed

printf '>>>???' | base64                                 # Pj4+Pz8/
printf '>>>???' | base64 | tr '+/' '-_' | tr -d '='      # Pj4-Pz8_

tr '+/' '-_' maps characters one-to-one (+→-, /→_), and tr -d '=' deletes the padding. In the JWT lessons you'll write this as a reusable function.