Add as development dependencies gems extracted from Ruby stdlib

Specs are currently failing on `master` on my local machine when I try
to run specs with
`BUNDLE_GEMFILE=/home/david/code/paper_trail/gemfiles/rails_6.1.gemfile
bundle exec rspec`. This isn't yet reflected in any CI builds on
`master`, but I believe that it would be, if a build were to be
triggered on `master`. The same error is also occurring in recent PRs
that have been put up (e.g. [this PR][1] with [this failed build][2] and
[this PR][3] with [this failed build][4]).

[1]: paper-trail-gem#1509
[2]: https://github.com/paper-trail-gem/paper_trail/actions/runs/13835939334/job/38733099092?pr=1509
[3]: paper-trail-gem#1511
[4]: https://github.com/paper-trail-gem/paper_trail/actions/runs/13916744592/job/38941085149?pr=1511

I'm not sure why these errors have started occurring now, without any
changes in `master` since [the last commit on `master`][5] that passed
all checks. Maybe `bundler` changed its behavior, and we are pulling in
a new/recent version of `bundler` with some relevant behavior change?

[5]: paper-trail-gem@94e9c0d

But, anyway, the errors seem to be caused by the extraction of various
gems from the Ruby standard library into standalone gems, specifically:

1. `bigdecimal`
1. `drb`
1. `logger`
1. `mutex_m`

This change adds those gems as development dependencies of
`paper_trail`.

Additionally, this change adds a `require "logger"` statement to
`spec/spec_helper.rb`. This isn't needed for the other gems (I'm
guessing because `activerecord` and/or `actionpack` have `require`
statements for `bigdecimal`, `drb`, and `mutex_m`?), but it is needed
for `logger`; without it, we get an error when trying to boot specs
about `NameError: uninitialized constant
ActiveSupport::LoggerThreadSafeLevel::Logger`.

Author: davidrunger

paper_trail.gemspec

@@ -56,6 +56,13 @@ has been destroyed.
   # PT supports request_store versions for 3 years.
   s.add_dependency "request_store", "~> 1.4"
 
+  # The following gems have been extracted from the Ruby stdlib to gems, and we
+  # must manually include them here in order for specs to run.
+  s.add_development_dependency "bigdecimal", "~> 3.1"
+  s.add_development_dependency "drb", "~> 2.2"
+  s.add_development_dependency "logger", "~> 1.6"
+  s.add_development_dependency "mutex_m", "~> 0.3.0"
+
   s.add_development_dependency "appraisal", "~> 2.5"
   s.add_development_dependency "byebug", "~> 11.1"
   s.add_development_dependency "ffaker", "~> 2.20"

spec/spec_helper.rb

@@ -60,7 +60,8 @@
 Bundler.setup
 
 # Then, the chosen components of Rails would be loaded. In our case, we only
-# test with AR and AC.
+# test with AR and AC. We require `logger` because `active_record` needs it.
+require "logger"
 require "active_record/railtie"
 require "action_controller/railtie"