feat(proguard): A/B test new Symbolicator implementation

[loewenheim]

Based upon #66764.

This adds a new rate option "symbolicator.proguard-processing-ab-test" that causes proguard events to be processed using Symbolicator in addition to the existing Python method and the results to be diffed.

[Swatinem]

lgtm. though this needs a rebase on master :-)

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ AttributeError: 'NoneType' object has no attribute 'get' sentry.tasks.store.process_event View Issue
• ‼️ TypeError: 'NoneType' object is not subscriptable sentry.tasks.store.process_event View Issue
• ‼️ TypeError: 'NoneType' object is not iterable sentry.tasks.store.process_event View Issue

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