Summary:
This diff adds support for PG's timestamptz type as the clone time parameter when cloning a
database as of a point in time:
```
CREATE DATABASE db2 TEMPLATE db1 AS OF '2024-09-10 21:52:46.782360';
```
The Unix microseconds timestamp format is still supported.
Jira: DB-12773

Test Plan: `./yb_build.sh release --cxx-test integration-tests_minicluster-snapshot-test --gtest_filter *CloneYsqlSyntax/*`

Reviewers: mhaddad, kfranz

Reviewed By: kfranz

Subscribers: jason, yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D42966