Drop use of base64 gem and warn about invalid base64 input

[anakinj]

Description

This pull request removes the dependency to the base64 gem. Motivation for this is the warnings emitting if base64 is not in the Gemfile/gemspec of the component running it, this is something that is a thing in Ruby 3.3 forward(ref). Another approach would be to list base64 as a dependency to in the .gemspec, but I kinda like the control of the Base64 handling. Maybe in the future we can remove the custom logic.

A bit of base64 history:

#483
#551

This approach also allows nicer way to deprecate support for the looser base64 that is baked into RFC 2045

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

[anakinj]

I changed my mind. Think it's better to add the base64 gem as a dependency.