Proposed fix for Optional RestartSequenceOperation.StartValue #26560

[dmihaita]

Modified the RestartSequenceOperation.StartValue's type from long to Nullable and adapted the dependencies.

Closes #26560

[dnfadmin]

All CLA requirements met.

[roji]

Thanks for the PR!

Can you please add two tests in MigrationsTestBase for restarting a sequence with and without a value (seems like we're missing value there)? You can look at Alter_sequence_increment_by as a basis for that. These would need to be overridden in MigrationsSqlServerTest to assert on the actual SQL generated, and also in MigrationsSqliteTest, which should simply assert not supported (since SQLite doesn't support sequences).

If you're having trouble figuring it out, let me know.

If this is

[dmihaita]

Suggestions applied.
Adding the two unit tests revealed that the functionality was not complete ... In order to manage to pass the RESTART without WITH clause, I had to add a new property to ISequence interface (InternalStartValue), and use it only for RestartSequenceOperation, on Diff. Not sure it's the most proper way to establish if the startValue was specified or not.

[dmihaita]

I think the scope of this issue was covered with the latest commit.

Reverted all changes to Sequence and related.
Adjusted the unit tests (removed the unit test that checked the operation creation through diff, added two unit-tests that cover the query generation based on restart operation definition)

Feedback addressed