Support value generation with converters

[ajcvickers]

Current behavior is to throw by default if a converter is associated with a property that will use value generation
However, a value generator to use can be specified:

• On the property
• On the type mapping
• On a converter

A client-side generator is added to the Guid converters so that Guid key scenarios still work. We should find similar "safe" cases and make those work as well.

Original PR: #11479

[ajcvickers]

Consider the Oracle case in #11559 when looking into this.

[ajcvickers]

Note: when working on this, also consider #11970