ref(transport): move crash-time disk dumping into transport

[jpnurmi]

Let crash backends suspend the transport's network submission and automatically route crash-time envelopes through its disk-dump path.

This removes the need for allocating fallback disk transports in the backends, and more importantly, prepares for the upcoming thread-pool telemetry module, which cannot safely drain the transport queue from a crash handler (see #1883 (comment) and #1883 (comment) for more details). This change abstracts away the problem so that callers no longer need to care whether the SDK has already started dumping in-flight envelopes to disk.

Before (pseudo)

void backend_except_func()
{
    // ...
    sentry_transport_t *disk_transport = sentry_new_disk_transport(options->run);
    sentry__capture_envelope(disk_transport, envelope, options);
    sentry__transport_dump_queue(disk_transport, options->run);
    sentry_transport_free(disk_transport);
}

After (pseudo)

void backend_except_func()
{
    sentry__transport_suspend(options->transport);
    // ...
    sentry__capture_envelope(options->transport, envelope, options);
}

#skip-changelog (internal refactor)

[codecov]

Codecov Report

❌ Patch coverage is 75.86207% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.87%. Comparing base (add0ebc) to head (6883ac0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1938      +/-   ##
==========================================
+ Coverage   75.79%   75.87%   +0.08%     
==========================================
  Files          93       93              
  Lines       22167    22156      -11     
  Branches     3944     3945       +1     
==========================================
+ Hits        16801    16811      +10     
+ Misses       4483     4464      -19     
+ Partials      883      881       -2     

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[JoshuaMoelans]

very nice refactor 👌