Refactor ReactorUtils into its own sentry-reactor module

[lcian]

📜 Description

Refactor ReactorUtils into its own sentry-reactor module

💡 Motivation and Context

Closes #3144

💚 How did you test it?

Unit tests and manual tests with the webflux samples.

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

• Do we need to add this module to the Gradle and Maven plugins?
• Any extra steps required for release of the SDK and/or the plugins?
• Do we need a major for this? Should we keep the old classes and mark them as deprecated instead, so that we can remove the old ones and swap to these on the next major?
• Update the docs if needed

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Refactor `ReactorUtils` into its own `sentry-reactor` module ([#4155](https://github.com/getsentry/sentry-java/pull/4155))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against 79bcd82

[lcian]

The commented out tests in SentryReactorUtilsTest are currently failing and I don't understand why.
The setup we do in the setup function should be sufficient to enable context propagation and use our SentryReactorThreadLocalAccessor.

Edit: I was not initializing the SDK :) Also the ThreadLocalAccessor can be set up with SPI instead.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	369.81 ms	481.23 ms	111.42 ms
Size	1.58 MiB	2.21 MiB	640.27 KiB

Previous results on branch: lcian/ref/reactor-as-module

Startup times

Revision	Plain	With Sentry	Diff
4bbe412	423.04 ms	466.40 ms	43.36 ms

App size

Revision	Plain	With Sentry	Diff
4bbe412	1.58 MiB	2.21 MiB	640.27 KiB

[adinauer]

Not fully done with reviewing the PR, so I'm just sending the comments I have already.
Also I opened a PR to fix some things: #4160

[adinauer]

Other things that might be missing from this PR:

• .craft.yml (we can only do this once the sentry-release-registry entry exists and we released this once)
• bug_report_java.yml should allow selecting the module for bug reports
• a README in the new module would be great
• root README has a list of modules where we should add this
  • there's also some missing there, we should add those as well, e.g. sentry-opentelemetry-agentless and sentry-opentelemetry-agent-spring

We need manually add this new module to sentry-release-registry after releasing this new module but we can already prepare the PR there beforehand.

Docs would also be great.