Summary:
Added runtime conf flag for eanbling off-cluster PITR feature. The flag is
`yb.ui.feature_flags.off_cluster_pitr_enabled`.

Test Plan: No tests

Reviewers: kkannan

Reviewed By: kkannan

Subscribers: svc_phabricator, yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D40176