fix: ruby 3.4 dropped base64 and mutex_m from the StdLib

```
/home/runner/work/json_api_client/json_api_client/gemfiles/vendor/bundle/ruby/3.4.0/gems/activesupport-6.0.6.1/lib/active_support/notifications/fanout.rb:3: warning: mutex_m was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add mutex_m to your Gemfile or gemspec to silence this warning.

/home/runner/work/json_api_client/json_api_client/gemfiles/vendor/bundle/ruby/3.4.0/gems/webmock-3.5.1/lib/webmock/util/headers.rb:1: warning: base64 was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.
```

Author: LeipeLeon

json_api_client.gemspec

@@ -23,6 +23,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency "webmock", '~> 3.5.1'
   s.add_development_dependency "mocha"
   s.add_development_dependency "appraisal"
+  s.add_development_dependency "base64"
+  s.add_development_dependency "mutex_m"
 
   s.license = "MIT"