Support runtime property attributes via TypeDescriptor

[warappa]

This adds support for property attributes configured via TypeDescriptor.

The code for PropertyOverridingTypeDescriptor is taken from StackOverflow as stated in the comment.

This should fix #52.

[DamianEdwards]

Thanks very much for this. A few small requests.

[warappa]

BTW: Is the dependency on version 5.0.0 of System.ComponentModel.Annotation for .NET Standard 2.0 OK, or should it be a lower version?

[DamianEdwards]

BTW: Is the dependency on version 5.0.0 of System.ComponentModel.Annotation for .NET Standard 2.0 OK, or should it be a lower version?

This is fine. It's the latest version and supports netstandard2.0.

Feel free to bump the version in src/Directory.Build.props to 0.9.0 too.

[DamianEdwards]

Feel free to bump version to 0.9.0.

[warappa]

Please don't yet merge, I think I found a double-report of validation errors for normal attributes.

[warappa]

TypeDescriptor seems to also report the normal attributes.
The weird thing is, that with a static Required attribute there is no double-report. But with a MaxLength attribute it was reported twice.

Anyway, with the previous commit the static attributes are now only considered once.

[DamianEdwards]

Required has special treatment in Validator to allow for early exit if it fails so my guess is that's what caused the behavior you saw.

[DamianEdwards]

@warappa is this ready to go then?

[warappa]

@DamianEdwards Yes, it's ready