Add assertions for net6.0 DateOnly/TimeOnly

[jnyrup]

Description

.NET 6 introduced two new interesting time structures, DateOnly and TimeOnly, see this PR.

Currently one can workaround the missing APIs in Fluent Assertions by converting DateOnly to DateTime and TimeOnly to TimeSpan.

new DateOnly().ToDateTime(TimeOnly.MinValue).Should();
new TimeOnly().ToTimeSpan().Should();

It would be nice if we had native APIs for asserting on DateOnly and TimeOnly.
I believe most assertions can simply be copied from the [Nullable]DateTimeAssertions and [Nullable]SimpleTimeSpanAssertions.

[julealgon]

@MullerWasHere can you confirm that you are working on this issue?

[MullerWasHere]

@julealgon I was working on it previously, haven't progressed much and I'm lacking time. If you want you can open a PR for it.

[iliashkolyar]

@jnyrup
Wondering regarding this one.
Shouldn't there be some prerequisite PR that adds net6.0 support to all relevant projects and builds before being able to add support to these new time structures?

[jnyrup]

My initial thought was that adding .NET 6 could be done as part of adding these assertions, as there otherwise is no/little value in adding the extra target framework.

[iliashkolyar]

Well, I tried to do so but it wasn't that straightforward.
There are a several compilation flags that check the TargetFramework and Include/Remove dependencies accordingly (like Common/NullConfigurationStore.cs). These are spread out across multiple projects, and the code analyzers are also configured via these flags.
There are several obsolete methods that fail compilation after the addition of the net6.0 framework such as BinaryFormatter.Serialize(Stream, object) and BinaryFormatter.Deserialize(Stream) (https://aka.ms/binaryformatter).

I think that these changes are not in scope for this specific feature implementation and should probably be performed by someone with better familiarity with the project.
What do you think?

[jnyrup]

Oh, thought it would be more straight forward.
I'll take a stab at adding .NET 6.

[jnyrup]

@iliashkolyar Fluent Assertions now also target net6.0.