Zabbix: Add options --with-mysql and --agent-only

- Build with PostgreSQL support by default.
- Allow to use MySQL optionally when installing `--with-mysql`.
- Prefer brewed over shipped DB, so switching between them is easier.
- Install only relevant schema for the selected DB.
- Allow to build only the Zabbix Agent with `--agent-only`.

Closes #16260.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>