Summary:
This diff adds gflag `enable_pg_anonymizer` to enable / disable the
postgresql anonymizer extension.

The main reason to do this is the ease of maintenance of a boolean
flag over ysql_pg_conf_csv. If the user makes a mistake while updating
ysql_pg_conf_csv, postmaster goes into a crash loop. Converting this
from a GUC to a gFlag would prevent this.
Jira: DB-16185

Test Plan:
./yb_build.sh --cxx-test pg_anonymizer-test
./yb_build.sh --java-test TestPgRegressThirdPartyExtensionsPostgresqlAnonymizer

Reviewers: jason, hsunder

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D43187