gemspec: add runtime dependencies on base64, bigdecimal

[olleolleolle]

Description

This PR adds dependencies in the Gemfile on gems which are referred to in the code, but which in Ruby 3.4 will not be bundled with Ruby.

If absent, and relying on the bundled-with-Ruby gems, these gems emit warnings in our test suite.

See #1757

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• RDoc comments have been updated
• CHANGELOG.md has been updated

[olleolleolle]

The test that fails (before and after this PR) is about the CCK, cucumber compatibility kit.

  1) Cucumber Compatibility Kit 'attachments' example generates valid message types
     Failure/Error: DEFAULT_FAILURE_NOTIFIER = lambda { |failure, _opts| raise failure }

       expected collection contained:  [:attachment, :attachment, :attachment, :attachment, :attachment, :attachment, :attachment, :attachme... :test_step_started, :test_step_started, :test_step_started, :test_step_started, :test_step_started]
       actual collection contained:    [:attachment, :attachment, :attachment, :attachment, :attachment, :attachment, :attachment, :attachme... :test_step_started, :test_step_started, :test_step_started, :test_step_started, :test_step_started]
       the missing elements were:      [:attachment, :step_definition]
     Shared Example Group: "cucumber compatibility kit" called from ./compatibility/cck_spec.rb:13
     # ./compatibility/support/shared_examples.rb:22:in `block (2 levels) in <top (required)>'

[luke-hill]

@olleolleolle not forgotten about this. I know what needs fixing and where, will be doing it soon

[luke-hill]

It would be good to get a grasp on where these are required, and if we can push any deps into the core engine, as I'm trying to get v10 to move most of the requirements into core.

[luke-hill]

Update - not forgot bout this @olleolleolle - Next version is v10 which has a few breaking version changes. This will be in that.