Include package name in citus_version() output

`SELECT version()` output contains some information about the package name and source but `SELECT citus_version()` does not. I think it can be helpful to know the exact package name and source in the output (if that is available)

```
postgres# SELECT citus_version();
┌────────────────────────────────────────────────────────────────────────────────────┐
│ citus_version │
├────────────────────────────────────────────────────────────────────────────────────┤
│ Citus 9.2.4 on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit │
└────────────────────────────────────────────────────────────────────────────────────┘
(1 row)

postgres# SELECT version();
┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ version │
├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│ PostgreSQL 12.2 (Debian 12.2-2.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
(1 row)
```

The part `(Debian 12.2-2.pgdg100+1)` indicates that the PostgreSQL package is from a PGDG Debian repository that runs PG12.2 and the 2nd patch release.

I do not yet know about the `100+1` part at the end.

Related: #3230