Add `SentryAttribute.flattened`

[adinauer]

:scroll: Description

Add SentryAttribute.flattened to allow flattening objects into separate key/value pairs of simple types. e.g.:

SentryAttribute.flattened("p", new Point(10, 20)) will add multiple attributes under the hood: p.x=10 and p.y=20.

This is currently done only for a single level, however we can expand it to more depth if needed.

:bulb: Motivation and Context

Provide API that takes care of flattening without having to manually set each attribute.

:green_heart: How did you test it?

:pencil: 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.

:crystal_ball: Next steps

[github-actions[bot]]

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

	Messages
:book:	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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

- Add `SentryAttribute.flattened` ([#4417](https://github.com/getsentry/sentry-java/pull/4417))

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

Generated by :no_entry_sign: dangerJS against 628254d5ef20f773fa27e63d59788b49c1f63c39

[github-actions[bot]]

Performance metrics :rocket:

	Plain	With Sentry	Diff
Startup time	438.08 ms	479.96 ms	41.88 ms
Size	1.58 MiB	2.08 MiB	510.55 KiB

Baseline results on branch: feat/better-attributes-api

Startup times

Revision	Plain	With Sentry	Diff
fe70d60562496bf90935ea5bbac7886f560cb75e	401.20 ms	545.47 ms	144.27 ms

App size

Revision	Plain	With Sentry	Diff
fe70d60562496bf90935ea5bbac7886f560cb75e	1.58 MiB	2.08 MiB	510.53 KiB