fix: UTF-8 retention problems with native SentryEvent properties

[limbonaut]

This PR fixes issues with retaining UTF-8 encoding in native SentryEvent implementation, discovered while working on tests in #326. The problem is that getters should be using String::utf8() where we reasonably expect UTF-8. Extracted from #326.

• Fixes UTF-8 not preserved in some API calls on Linux and Windows #336
• Partially implements Add tests for UTF-8 retention across API calls that deal with strings #337, more are coming in wip: Add integration tests #326

[limbonaut]

This replaces multiple tests removed below.

[jpnurmi]

Are there any recursive dict/array conversions or such where the same problem could occur?

[limbonaut]

@jpnurmi Yes, we convert Godot types to internal SDK types recursively, but not the other way around (at least not yet). We’ll be testing it in one of the future PRs. Here’s a work-in-progress glimpse: c5969ba