feat: read and capture envelopes

[jpnurmi]

Expose minimal envelope API for crash reporters / feedback handlers. Extracted from #1303.

1. allow reading an envelope from the disk
2. expose envelope headers (DSN and event ID)
3. allow submitting the envelope to Sentry

Semi-pseudo example:

int main(int argc, char *argv[])
{
    // 1.
    sentry_envelope_t *envelope = sentry_envelope_read_from_file(argv[1]);

    // 2.
    sentry_value_t dsn = sentry_envelope_get_header(envelope, "dsn");
    sentry_value_t event_id = sentry_envelope_get_header(envelope, "event_id");

    sentry_options_t *options = sentry_options_new();
    sentry_options_set_dsn(options, sentry_value_as_string(dsn));
    sentry_init(options);

    /* ... */

    // 3.
    sentry_capture_envelope(envelope);

    sentry_close();
}

Minimal but complete feedback handler used for integration tests: feedback.c

[github-actions]

	Messages
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[vaind]

Looks alright to me except for a few potential issues below.

[jpnurmi]

Thanks, good catches! 🙏 I added a bunch of invalid test cases, and also the examples listed at https://develop.sentry.dev/sdk/data-model/envelopes/#full-examples

[vaind]

@sentry review

[vaind]

Thanks for the changes, the code is much more defensive now

[JoshuaMoelans]

LGTM! do you think it makes sense to merge this before merging in the int64 PR (which is currently tied to Logs, but could be merged separately before fully finishing the Logs PR), or should we already have int64 values in this PR?

[jpnurmi]

do you think it makes sense to merge this before merging in the int64 PR (which is currently tied to Logs, but could be merged separately before fully finishing the Logs PR), or should we already have int64 values in this PR?

I'd vote for merging int64 support separately. It's needed for multiple things i.e. logs and envelope parsing, but not strictly related to either of them. 😉

[jpnurmi]

I'll fix the TODOs when int64 support is available. It's not urgent because Sentry only supports ~100MB payloads at the moment.