feat: Take DSN use minidump url

num_breadcrumbs++ and the file write that follows it were not
protected against concurrent calls from multiple threads.  Two threads
calling sentry_add_breadcrumb simultaneously could observe the same
rotation boundary (first_breadcrumb == true) and both overwrite the
breadcrumb file, or write interleaved msgpack to it.

Add a std::mutex (breadcrumb_mutex) to wer_state_t that serialises
the counter read-modify-increment and the subsequent path write as a
single critical section.  This mirrors the mutex-based protection used
by the in-proc and Crashpad backends.

Fixes audit finding: Bug #2 (breadcrumb thread-safety).