ref(attachments): rework to be `sentry_value_t`-based

[JoshuaMoelans]

Follow-up to #1934 as brought up by @jpnurmi ; the fix we provided there works for now, but looking forward we'd ideally end up with a more robust attachments-handling system by implementing the following:

• replace heap-allocated sentry_attachment_t objects with sentry_value_t-backed value types,
• replace global scope mutex with fine-grained read-write locks, and
• replace backend flush functions with scope observers (see feat(internal): add scope observer #1839 )

[linear-code]

NATIVE-211