Add dependency to base64 gem

[yahonda]

because base64 is bundled gem since Ruby 3.4.0dev.

ruby/ruby#9550
https://bugs.ruby-lang.org/issues/20187

Here is the LoadError to use rubyntlm using Ruby 3.4.0dev.
https://github.com/yahonda/httpclient/actions/runs/7695505773/job/20968523567#step:4:13

/home/runner/work/httpclient/httpclient/vendor/bundle/ruby/3.4.0+0/gems/rubyntlm-0.6.3/lib/net/ntlm.rb:39: warning: base64 was loaded from the standard library, but is not part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.
/home/runner/.rubies/ruby-head/lib/ruby/3.4.0+0/bundled_gems.rb:74:in `require': cannot load such file -- base64 (LoadError)

[pcai]

@yahonda thank you.