Add generator for initializer generation

[st0012]

Usage

$ bin/rails generate sentry 
$ bin/rails generate sentry --dsn my_dsn

Implements #2218

[codecov]

Codecov Report

Merging #2286 (d9c2ae7) into master (0b318c0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2286      +/-   ##
==========================================
- Coverage   98.72%   98.72%   -0.01%     
==========================================
  Files         200      202       +2     
  Lines       13253    13282      +29     
==========================================
+ Hits        13084    13112      +28     
- Misses        169      170       +1     

Components	Coverage Δ
sentry-ruby	99.10% <ø> (ø)
sentry-rails	97.37% <100.00%> (+0.03%)	⬆️
sentry-sidekiq	96.98% <ø> (ø)
sentry-resque	96.76% <ø> (-0.33%)	⬇️
sentry-delayed_job	98.91% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-rails/lib/generators/sentry_generator.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/generator_spec.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes