[ticket/10202] Add database schema for phpbb_config_db_text.

PHPBB3-10202
[ticket/10202] Adjust method names to guidelines.

PHPBB3-10202
[ticket/10202] Add tests for phpbb_config_db_text.

PHPBB3-10202
[ticket/10202] Define phpbb_config_db_text as a service.

PHPBB3-10202