Add `base64` as a runtime dependency

[orien]

Context

Webmachine uses the base64 library from the Ruby standard library:

https://github.com/webmachine/webmachine-ruby/blob/45d7513aaf082961be85de024d76f8bbf06a0f98/lib/webmachine/decision/flow.rb#L3

In the forthcoming Ruby 3.4, the base64 library will be come a 'bundled' library. It must be included in gem sets to be used with Bundler.

In Ruby 3.3 a deprecation warning is shown:

lib/webmachine/decision/flow.rb:3: warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.

Example: https://github.com/webmachine/webmachine-ruby/actions/runs/7955436555/job/21714422307?pr=276#step:5:5

Change

Add base64 as a runtime dependency in the gemspec.