Allow default value check in value generation to be customized

[ajcvickers]

Currently we generate a value for a property on Add when it is marked as needing value generation in metadata and the current value is the default for the type of property. It should be possible to customize the latter check to change the this sentinel value, make it a more complex check (e.g. negative numbers) or so that values are always generated regardless of the current value. This is probably best done with a Func in the model, but note this requires we implement the ability to annotate a model with code snippets.

[ajcvickers]

See also the discussion in #15070 (comment)

Use of a different sentinel value could be useful here if it is set to the same as the default value.

• This would mean that if the value is set to default, then it would not be inserted, since this value is the same as the sentinel. But it would get the default value from the database as expected.
• However, this would be a breaking change since it would insert the CLR default rather than using the default from the database
• We could even consider automatically making the default value the same as the sentinel value if it has been set.

[roji]

This could be more relevant with non-nullable reference types. For example, non-nullable string properties can no longer default to null (unless bang is used), so it makes sense for many cases to initialize it e.g. to an empty string. But EF still uses null as the uninitialized sentinel value (to trigger value generation).