Support worker_spi to execute the function dynamically.

Hi,

While I'm working on the thread[1], I found that the function of
worker_spi module fails if 'shared_preload_libraries' doesn't have
worker_spi.

The reason is that the database name is NULL because the database name
is initialized only when process_shared_preload_libraries_in_progress
is true.

```
psql=# SELECT worker_spi_launch(1) ;
2023-07-20 11:00:56.491 JST [1179891] LOG: worker_spi worker 1
initialized with schema1.counted
2023-07-20 11:00:56.491 JST [1179891] FATAL: cannot read pg_class
without having selected a database at character 22
2023-07-20 11:00:56.491 JST [1179891] QUERY: select count(*) from
pg_namespace where nspname = 'schema1'
2023-07-20 11:00:56.491 JST [1179891] STATEMENT: select count(*) from
pg_namespace where nspname = 'schema1'
2023-07-20 11:00:56.492 JST [1179095] LOG: background worker
"worker_spi" (PID 1179891) exited with exit code 1
```

In my understanding, the restriction is not required. So, I think it's
better to change the behavior.
(v1-0001-Support-worker_spi-to-execute-the-function-dynamical.patch)

What do you think?

[1] Support to define custom wait events for extensions
https://www.postgresql.org/message-id/flat/b9f5411acda0cf15c8fbb767702ff43e%40oss.nttdata.com

Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION