Add support for PGOPTIONS environment variable

Nobody has claimed this yet.

asyncpg supports an explicit server_options=... dict connection method parameter, but does not seem to populate its value from the PGOPTIONS environment variable. This is a feature request for it to do so.

Specifically, `asyncpg/connect_utils.py` reads user, host, port, database, password, and SSL information with calls like `os.getenv('PGUSER')`, `os.environ.get('PGHOST')`, etc. There is no corresponding call for reading PGOPTIONS.

I would like to set the environment variable `PGOPTIONS='-c default_transaction_read_only=on -c statement_timeout=120s'` and have asyncpg interpret its values as server_settings the way libpq does. For instance, the below works for psql:

```
PGOPTIONS='-c statement_timeout=1s' psql -c "SELECT pg_sleep(2)"
ERROR: canceling statement due to statement timeout
```

but a simple asyncpg script running the same ignores the statement_timeout since it never reads the value of PGOPTIONS.

```
$ PGOPTIONS='-c statement_timeout=1s' ./test_asyncpg_timeout.py
SELECT 1
```

https://www.postgresql.org/docs/current/libpq-envars.html

Start in asyncpg/connect_utils.py, where the existing PG environment variables are read, and compare the requested behavior with the PostgreSQL libpq-envars documentation. Run the provided test_asyncpg_timeout.py reproduction with PGOPTIONS set; done means PGOPTIONS values are interpreted as server settings and the statement_timeout example behaves like psql.

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