Drop runtime dependency on base64

[Earlopain]

#1003 added base64 as a runtime dependency. It's easy enough to replace, the base64 is for the most part a wrapper around the pack/unpack methods.

Also see:

• Remove runtime dependency on base64 lostisland/faraday#1541
• Remove runtime dependency on base64 bblimke/webmock#1046
• [Fix #12310] Drop base64 gem from runtime dependency rubocop/rubocop#12313
• Remove base64 dependency rack/rack#2110
• etc.

I have made it a dev dependency since one test file uses it in a bunch of places and replacing all that doesn't make the code much better.

[dentarg]

Why? The abstraction is there for a reason!