SQL function to access to `creating_extension`

Hi,

I propose to introduce `pg_creating_extension()` function that would return
the OID of the extension being currently created (or `null` if none is).

The core motivation for it is to be able to establish data provenance in
tables created by extensions to be used in `pg_extension_config_dump`
configuration. This way, even if a secondary extension inserts data, it can
still be excluded from the dump by tracking data provenance in a column
with a default. Something like

```
create table my_config (
--- ...
extension_provenance oid default pg_creating_extension()
)
```

This would allow for a generalized exclusion in pg_extension_config_dump.

I've attached a draft patch for this simple function. I am happy to
finalize it with tests and documentation if there is a consensus on the
shape.

--
Founder at Omnigres
https://omnigres.com