Miscellaneous cleanup/throw helpers

[Rob-Hague]

These are some random changes I've accumulated in some branches, which I've extracted out to reduce diffs & conflicts. They are roughly:

Add a reference to PolySharp to allow nullable attributes and CallerArgumentExpressionAttribute etc.
Add a ThrowHelpers class
Add #nullable to PrivateKeyFile and cleanup PrivateKeyFileTest
Also change around some of the newly added keys - I have a use for this with certificate support.

[Rob-Hague]

Ok to get a sign-off here?

[drieseng]

Not possible to fix these?

[Rob-Hague]

They are coming from string.IsNullOrEmpty and friends which are not annotated for nullable analysis on lower targets

[drieseng]

Why was this one reported? Don't have time to check myself now.

[Rob-Hague]

It comes when one wants to provide a value for a parameter marked with [CallerExpressionAttribute], e.g.

publicTimeSpanTimeout{get{return_timeout;}set{// EnsureValidTimeout(this TimeSpan timeSpan, [CallerArgumentExpression(nameof(timeSpan))] string? paramName = null)value.EnsureValidTimeout(nameof(Timeout));_timeout=value;}}

I would probably suppress this one at the call sites, but #1480 will also bring more places where we don't want it (from Debug.Assert)

[Rob-Hague]

Thanks

[scott-xu]

Also change around some of the newly added keys - I have a use for this with certificate support.

How about consolidating the keys. e.g. one key-pair per algorithm (and key size) , but with different format and different encryption for private key? It will be easier for integration test.

[Rob-Hague]

It seems OK how it is - it was a bit of a pain to set everything up for #1498 so I would rather like to avoid changing it