Add transaction source annotation

[sl0thentr0py]

See https://develop.sentry.dev/sdk/event-payloads/transaction/#transaction-annotations for spec.

This is necessary going forward because we need to separate high cardinality / low quality (for us, only raw urls from rack) to enable ingestion to do metrics aggregation without blowing up.

Note: This is independent in principle, but I'm basing it off the baggage changes since all of this needs to go out together anyway and some of the Transaction constructors changed and would cause merge conflicts otherwise.

closes #1866

[codecov-commenter]

Codecov Report

Base: 98.42% // Head: 98.43% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (d1768b6) compared to base (8fae12f). Patch coverage: 97.77% of modified lines in pull request are covered.

Additional details and impacted files

@@                        Coverage Diff                        @@
##           neel/baggage-dsc-continuation    #1902      +/-   ##
=================================================================
+ Coverage                          98.42%   98.43%   +0.01%     
=================================================================
  Files                                150      150              
  Lines                               9183     9195      +12     
=================================================================
+ Hits                                9038     9051      +13     
+ Misses                               145      144       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/event.rb	98.76% <ø> (ø)
sentry-ruby/lib/sentry/rake.rb	65.00% <0.00%> (ø)
...entry-delayed_job/lib/sentry/delayed_job/plugin.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/action_cable.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/active_job.rb	100.00% <100.00%> (ø)
...entry-rails/lib/sentry/rails/capture_exceptions.rb	100.00% <100.00%> (+3.44%)	:arrow_up:
...y-rails/lib/sentry/rails/controller_transaction.rb	100.00% <100.00%> (ø)
sentry-resque/lib/sentry/resque.rb	97.22% <100.00%> (+0.07%)	:arrow_up:
sentry-ruby/lib/sentry/rack/capture_exceptions.rb	95.45% <100.00%> (ø)
sentry-ruby/lib/sentry/scope.rb	100.00% <100.00%> (ø)
... and 7 more

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