Simplify API for flushing events

[mattjohnsonpint]

This is mostly just for convenience, simplifying usage and documentation.

• Move the FlushTimeout option to SentryOptions.

  • Previously this was defined separately on SentryAspNetCoreOptions and SentryNLogOptions only.
  • Note, this is not a breaking change, as those classes inherit from SentryOptions already.
  • We default to 2 seconds, except for NLog where we preserve the existing 15 second default.

• Add FlushAsync() APIs where we previously had FlushAsync(timeout).

  • When a timeout is not specified, the value from the FlushTimeout option is used.

• Added synchronous versions, Flush() and Flush(timeout). They are blocking sync-over-async, but that's what we've been doing already anyway. This reduces error and gets .Wait() or .GetAwaiter().GetResult() out of samples and docs.

• Updated existing usages throughout the solution

• Added tests