Distinguish cancellation from failure

[roji]

• Instead of QueryIterationFailed, SaveChangesFailed, and CommandError, we now emit QueryCanceled, SaveChangedAsync and CommandCanceled when we detect cancellation. The default log level for the cancellation events is Debug.
• Cancellation is detected by looking at the exception rather than checking whether the token has been triggered. This allows:
  • Avoid race conditions where a non-cancellation error is bubbling up just as the user triggers the token.
  • Detect cancellations not triggered via the cancellation token (e.g. triggered in the database directly). This is also why the sync execution path is covered.
• Unfortunately, SqlClient provides no way to detect cancellation exceptions (see Cancelling an async SqlClient operation throws SqlException, not TaskCanceledException SqlClient#26 (comment)). As a result, cancellation detection logic is in a new ExceptionDetector singleton service, overridden by SQL Server to check the cancellation token (so the two problems above exist for SQL Server).

Closes #19526

/cc @ajcvickers for logging infra rocket science 🚀🚀🚀

PS We can consider moving transient exception detection into the new ExceptionDetector as well. The default implementation could call the new DbException.IsTransient, the SQL Server implementation would just contain the logic currently in SqlServerTransientExceptionDetector etc.