Improvements to exception mechanism data

[mattjohnsonpint]

This PR address several items related to the exception.mechanism data sent to Sentry.

• Prevents an empty mechanism. Either exception.mechanism is omitted, or the exception.mechanism.type field will be set. Defaults the mechanism type to "generic" when not set by another method.
  Fixes Exception mechanism type unset #2288.

• Moves extra data added to System.Exception.Data to the exception.mechanism.data collection, instead of placing it in exception.extra. This places it near the exception in the issue details page, and renders it correctly.
  Fixes Move exception data to mechanism #2289.

• Adds descriptions to exception mechanisms for built-in integrations, and a new SetSentryMechanism extension method for convenience. Also adds a property for the synthetic field. Omits it unless true. The default is false.
  Fixes Unused mechanism fields #2290.

• Sets exception.mechanism.handled to true in the case of an exception that was thrown, caught, and manually captured. As an example:

  try
  {
      // ... something that throws an exception
  }
  catch (Exception ex)
  {
      // this will now be correctly marked as handled:true
      SentrySdk.CaptureException(ex);
  }

  Fixes Add handled:true when an exception has been handled #1480.

[mattjohnsonpint]

Note, many of these items are prerequisites for #2240 - coming soon with #2287.

[mattjohnsonpint]

Here are some screenshots of how this looks in Sentry:

Caught and handled exception:

Uncaught exception (hovering over the mechanism for description popup):

Captured exception that was never thrown: