Add Rails 8 example app

[st0012]

When investigating #2467, I wanted to see how sentry-rails would work with SolidQueue and other Rails 8 features, but there's no example apps to try. So this PR is the first step to add an Rails 8 example app to help us test and develop features for newer Rails apps.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.14%. Comparing base (1cd952b) to head (b0d2f3b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2512      +/-   ##
==========================================
- Coverage   98.16%   98.14%   -0.03%     
==========================================
  Files         128      128              
  Lines        4848     4848              
==========================================
- Hits         4759     4758       -1     
- Misses         89       90       +1     

Components	Coverage Δ
sentry-ruby	98.54% <ø> (ø)
sentry-rails	96.91% <ø> (-0.16%)	⬇️
sentry-sidekiq	97.09% <ø> (ø)
sentry-resque	92.85% <ø> (ø)
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

see 1 file with indirect coverage changes