Add code generation support for insertable / updatable Records and POJOs

[lukaseder]

There have been some interesting discussions in a previous issue:

• Discussion about improvements related to the type safety of CRUD operations #14566 (comment)

A common desire, especially in the kotlin ecosystem, is to have a more accurate reflection of nullability in some types of columns, which are nullable on insert, but non-nullable on read. This includes identity columns, for example. Numerous examples of users requesting changes here are:

• Code generator is annotating setters/getters of nullable columns in Record classes with @Nullable. #14786
• "Nullable" Kotlin generator is incorrect for columns with default values #14787
• Kotlin code generator ignoring isKotlinNotNullRecordAttributes = true or I misunderstand this feature. #14948
• Kotlin Code generator generated nullable property for "not null" database column with default value #14978
• Ensure PostgreSQL 'serial' Column Type Generates Non-Nullable Kotlin Variable #16593

jOOQ cannot accomodate these wishes out of the box, as it must be possible to create a POJO / Record containing a null value for an identity. It will become non-null only after storing the value (if it is fetched back from the server).

A thorough solution would be to offer multiple representations of the same record for multiple purposes:

• A representation for INSERT (obtained when calling ctx.newRecord(TABLE))
  • Defaulted columns, including identities, are always nullable (irrespective of NOT NULL constraints)
  • Non-insertable columns are absent
• A representation for UPDATE (obtained when fetching ctx.selectFrom(TABLE) or ctx.insertInto(TABLE).returning(), etc.)
  • NOT NULL columns are always non-nullable
  • Non-updatable columns are readonly

Other representations as suggested in #14566 won't be added for now:

• A readonly representation
• An all-nullable representation

This feature interacts with the suggested concept of insertable and updatable columns:

• Add support for insertable and updatable flags on columns #16005

Tasks

• Implement the code generation feature
  • It must be opt-in, default is the current behaviour
• Implement new runtime API. There's going to be a need for new TableRecord style marker interfaces, which offer the appropriate generics to allow for the appropriate change of behaviour with ctx.newRecord(), etc.

Caveats

• Current UpdatableRecord API can't model the state transition from an InsertableRecord to an UpdatableRecord: record.store() and record.insert() returns only the update count, not the new representation.

This issue needs a lot more thought before we can start implementing it