Added Sequence generation support

[mipo256]

Hey @schauder, @mp911de!

Hope you're doing great, I've redone the PR #1922 to make it work via a custom callback that is registered as a bean in AbstractJdbcConfiguration. Let me know what you think of that, thanks!

CC: Targeting issue #1923

[mp911de]

Hey, great to see a new contribution.

We discussed identifier generation a while ago and wanted explore pathways forward. IdentifierGenerator is a rather specific approach and hard to type (identifiers could be numbers, UUIDs, some random characters). We could explore a ValueGenerator, maybe even ValueGenerator<T> abstraction that is implemented by a sequence generator along with a @GeneratedValue(strategy = SEQUENCE | NATIVE) (NATIVE is what Statement.getGeneratedKeys returns) annotation that could be used on a per-property level.

The implemented SequenceValueGenerator<T extends Number> extends ValueGenerator<T> should encapsulate JdbcTemplate along with the SQL text to obtain the value.

[mipo256]

@mp911de So, basically, what we want to achieve is the following:

@Table
public class MyEntity {
    
    @Id
    @GeneratedValue(strategy = SEQUENCE)
    private Long id;
    
    @GeneratedValue(strategy = SEQUENCE)
    private Long version;
    
    @GeneratedValue(strategy = NATIVE)
    private Long someValue;
} 

Where we support sequence generation for various fields, along with generation via identitiy columns in database via NATIVE placeholder, am I correct?

[mp911de]

While not necessarily generating version (implied; as it would contradict our @Version handling) that is the underlying idea. Paging @schauder for visibility.

In the long run, I could also imagine some @ValueGenerator(UUIDValueGenerator.class) usage but that is a future idea.