poc: thread sampling

[bitsandfoxes]

wip

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🚫	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

### Features

- thread sampling ([#1721](https://github.com/getsentry/sentry-native/pull/1721))

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