Add ThrowAsync, ThrowExactlyAsync and NotThrowAsync

[krajek]

Closes #851.

[jnyrup]

The other exception assertions have the ThrowExactly extension located in AssertionExtensions.Actions.cs.
To provide the same set of assertions for the async version, ThrowExactlyAsync should be added.

It would be nice with a test exercising that ThrowAsync<TException> also works for derived types of TException, like When_subject_throws_subclass_of_expected_exact_exception_it_should_fail does.

[krajek]

@jnyrup I have just added ThrowExactlyAsync. Ready to merge from my side.

[krajek]

I have just completed fixes after the second review

[nerdpad]

Any plans of release this soon? I just started using FluentAssertion and this is exactly what I needed.

[jnyrup]

@nerdpad yes, we're also quite eager to get out a new release with all the new features, fixes and other improvements.
We're finishing up the last PRs for 5.5.0, see the remaining ones here https://github.com/fluentassertions/fluentassertions/milestone/12