Migrate sentry-sidekiq to AJ-first integration #2937

Part of #2932.

Let ActiveJob jobs on Sidekiq be handled by the ActiveJob integration, with sentry-sidekiq enriching them through the registry instead of reporting them itself. Plain Sidekiq workers, sentry-sidekiq-only installs and non-Rails apps keep the existing integration.

In 7.x this is opt-in (#3118), and opting in must not change anything we already send for these jobs (names, tags, contexts, retry reporting, producer spans); it can only add data. Because it's behind the opt-in, it can land across several PRs, with the opt-in enabled last.

Depends on #3117, #3119, #3122