Make existing model building conventions more extensible

[ajcvickers]

I have been attempting to create my own PropertyDiscoveryConvention, but it's difficult without copying almost all of the existing code in the convention, and creating my own helpers (#28854). Things would be easier in this and other cases if existing convention classes:

• Made appropriate private methods protected
• Split some of these methods up to allow more granular overrides
• Added pure template methods in useful places