Change some metadata extension methods into DIM properties and annotate for nullability

[roji]

@AndriySvyryd we can also move other extension methods to be properties on the interfaces if we want (especially internal ones).

[AndriySvyryd]

All public extensions should continue to be methods to avoid the breaking change, using properties doesn't give any significant enough advantage. Eventually we'll need to convert all of them to DIM - #19213

[roji]

@AndriySvyryd are you saying to want me to add back the extension methods, calling the new properties, to maintain binary backwards compat here? Or that you're against the properties altogether (although we want to do #19213 anyway)?

Re the advantage of properties, the main reason for introducing them was to be able to use the new [MemberNotNullWhen], which isn't usable on extension methods.

[AndriySvyryd]

@roji No, I'm saying that they should be methods with DIM. We don't necessarily need binary compat, but we should preserve compile compat.

[roji]

Will change.

[AndriySvyryd]

HasClrType can become a property, since it was pubternal before

[ghost]

Hello @roji!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[smitpatel]

(Removed auto-merge, I will add it back. Trying to get release/5.0 merged to main)

[AndriySvyryd]

(nit) Commit description is not accurate

[roji]

@AndriySvyryd fixed, @smitpatel no problem.