Allow omitting value generation via convention when scaffolding

[roji]

AnnotationsCodeGenerator currently allows to specify that a given annotation shouldn't be code-generated, since it is by-convention. However, the same isn't true of other, non-annotation settings such as value generation.

More concretely, PostgreSQL supports several methods of database-generated autoincrement values - serial (simple column backed by a sequence), and since PostgreSQL 10 also identity (with two variants, always and by default). NpgsqlValueGenerationStrategy has all these options and there are property builders for specifying them. The problem is that when scaffolding an identity column, in addition to the UseNpgsqlIdentityByDefaultColumn() property builder method, ValueGeneratedOnAdd() is generated, although the latter should be implied by the former.

This is obviously a very non-urgent issue.

[bricelam]

👍 We hit the same problem in #10183 where SQL Server adds index filters by convention.