Handle value generation when value conversion is being used

[ajcvickers]

Fixes #11010

Approach is:

• 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

[ajcvickers]

/cc @divega FYI

[divega]

@ajcvickers are there still further improvements (like the ones we discussed yesterday) that would make sense to track in an issue, or do you prefer to wait for feedback?

[ajcvickers]

@divega Probably worth having an issue to check for other "safe" generators we could add, but would rather wait until after 2.1 before we add any more anyway, so we could just wait for feedback instead.