feat: (POC) offline caching

[JoshuaMoelans]

Heavily Work In Progress/exploratory

Current Implementation

For now, we just add an option to write .dmp files to disk for Breakpad, allowing easier local debugging (since we only store up to 20 dumps per issue on Sentry, and to be useful these need debug symbols uploaded which during development doesn't happen).

Tried Approaches

'developer caching'

Although this seems versatile, this opens up more complexity (without adding that much value).

• For inproc/breakpad, we write .run directories into .sentry-native/cache upon restart (while processing old runs) instead of removing them. We periodically clean this folder based on size/age, similar to how Crashpad does this
  
• Additionally, whenever breakpad would write a minidump (which otherwise gets merged into the envelope), we keep this on disk too.
• We provide cache_max_age and cache_max_size for controlling the periodic cleanup.

TODO

• look into caching for Crashpad .dmp file (not getting stored in the .run directory)
  -> currently seems like one could look into .sentry-native (or other configured database_path) and look at the completed folder to see previous crash dump files.
• add tests
• think about offline caching for offline devices; we currently try to send, but if we fail there is no backup sending (except for Crashpad); in inproc/breakpad, we just hand the envelope to the transport and say GLHF... . How could we (/should we?) provide feedback for failed sends (and make these actionable)? -> maybe out of scope for this first implementation.

[github-actions]

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

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## Unreleased

### Features

- (POC) offline caching ([#1461](https://github.com/getsentry/sentry-native/pull/1461))

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