Add load_balance_hosts to spread connections across multiple hosts

[lesnik512]

With several hosts, connect() always tries them in the order given. Every connection, including every connection a pool opens, goes to the first host that accepts it. With target_session_attrs="read-only" or "prefer-standby" and several replicas, the first replica in the list gets all the load and the others sit idle.

This PR adds load_balance_hosts, the libpq option of the same name (PostgreSQL 16+):

• "disable" (default): hosts are tried in the order listed, as today.
• "random": on every connect() call the address list is shuffled before the usual host loop runs. Failover and target_session_attrs work as before; only the order changes. A pool calls connect() for each new connection, so pool connections end up spread across the hosts.

The value is resolved like target_session_attrs: keyword argument, then load_balance_hosts in the DSN, then the service file, then PGLOADBALANCEHOSTS, then "disable". Other values raise ClientConfigurationError. A load_balance_hosts DSN option is no longer sent to the server as a setting. Today it fails with UndefinedObjectError.

Why in asyncpg:

• Callers can't shuffle the hosts once and pass them in, because the order has to change for every connection. Today that takes a custom connect= for create_pool(), or an async_creator for SQLAlchemy's asyncpg dialect, that reorders the hosts before each call. Downstream projects already do this (for example modern-python/db-retry).
• The name, values, DSN key and environment variable are libpq's, the same way asyncpg already handles target_session_attrs, service, krbsrvname and gsslib. A DSN that load-balances with psql and psycopg works the same here.

libpq also shuffles the addresses a single host name resolves to. asyncpg leaves DNS resolution to the event loop's create_connection(), so this PR shuffles only the (host, port) entries.

Tests:

• TestConnectParams: default, DSN, keyword argument, environment variable, service file and invalid values.
• TestLoadBalanceHosts: with random the hosts are tried in the shuffled order and the order is drawn again on each connect(). With the default or disable, the listed order is kept and random isn't called.
• Full suite passes on Python 3.13 against PostgreSQL 18, and tests.test_connect on Python 3.10. flake8 and mypy are clean.