Support libpq compatible load_balance_hosts parameter

Hello @jackc,

I understand that we can use multiple hosts as described in the question #1184. This is supported by client applications relying on `plbpq` since PostgreSQL 10.

Do `pgx` suport/has a similar option as `libpq` to perform random load balance at client level added to PostgreSQL 16?

The new feature was added to be complaint with the existing JDBC implementation, introducing a new parameter to the connection string called `load_balance_hosts`, and new `env` variable `PGLOADBALANCEHOST`.

The option `load_balance_hosts` has two options, `Disable` where `libpq` will attempt to establish a connection using the host list provided(in order), with no load balancing, and `Random` providing attempt to connect to a randomly selected host from the list.

Do we have an option like that available currently in the project?

Thank you very much.

See:
- [PostgreSQL 16 brings Load Balancing support in libpq](https://www.mydbops.com/blog/postgresql-16-brings-load-balancing-support-in-libpq-psql)
- [Commit message from the PostgreSQL user list](https://www.postgresql.org/message-id/flat/PR3PR83MB04768E2FF04818EEB2179949F7A69@PR3PR83MB0476.EURPRD83.prod.outlook.com)

The issue names no files or tests; start by locating pgx's existing multi-host connection handling and connection-string and environment-option parsing. Compare that behavior with PostgreSQL 16's libpq load_balance_hosts options. Done means pgx has a documented compatible option that selects hosts in order or randomly as requested, with coverage for both modes.

Written by the indexing model from the issue text.