Remove base64 dependency

[Earlopain]

Avoids a warning with Ruby 3.3. The base64 gem will be removed as a default gem with Ruby 3.4, 3.3 will issue a warning.
Instead of adding this as a dependency to the gemspec, just use the available pack/unpack methods to which this gem is a simple wrapper.

• Remove base64 dependency rack/rack#2110
• [Fix #12310] Drop base64 gem from runtime dependency rubocop/rubocop#12313
• Remove runtime dependency on base64 newrelic/newrelic-ruby-agent#2378
• Remove runtime dependency on base64 lostisland/faraday#1541