Summary:
D34251 / 1821d2742511d9ce958c58108259f6665243f5dc added support for
security label statement but missed a few changes.

This diff fixes that and also enables dummy_seclabel security provider because
there are no working security label providers currently, without which there is
no way to test the changes.

The yb_pg_dummy_seclabel regress tests are added to confirm that the security
label statements indeed work as it should.
Jira: DB-15105

Test Plan: ./yb_build.sh --java-test TestPgRegressModulesDummySeclabel

Reviewers: jason, kramanathan, tnayak

Reviewed By: jason

Subscribers: mbautin, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41578