Add support for PGOPTIONS environment variable

还没有人认领这个 Issue。

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

从读取现有 PG 环境变量的 asyncpg/connect_utils.py 开始，并将所需行为与 PostgreSQL 的 libpq 环境变量文档进行对比。在设置 PGOPTIONS 的情况下运行提供的复现程序 test_asyncpg_timeout.py；当 PGOPTIONS 的值被解释为服务器设置，并且 statement_timeout 示例的行为与 psql 一致时，即表示完成。

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