feat(path): add sentry__path_copy for large attachments

[jpnurmi]

Normal attachments are safe after being embedded into the envelope during a crash. However, with the upcoming large attachments that are uploaded separately and not stored in the envelope to avoid ever reading large attachments into memory, we must make a copy of the large attachment to keep it safe. This PR adds an internal API for doing that.

• Normal attachments: safely embedded into envelopes during crashes ✅
• Large minidumps: safely written straight into the database directory ✅
• Other large attachments: arbitrary external files that cannot be safely referenced across sessions -> must be copied to keep the data safe ❌

Extracted from #1545

#skip-changelog (internal)

[github-actions]

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