Feature Request: show service name in prompt

I primarily use `pgcli service=svc_name` to start pgcli with everything defined in `~/.pg_service.conf`. I would like the ability for that service name to appear in the prompt. I currently use `\h` to show the hostname, but but our hosts are things like `psql-dept-tier-azflexdb-application`, and I'd much prefer to just see my `svc_name` in the prompt.

It looks like `prompt_dsn` is only provided to the `PGCli` class for the purpose of displaying in the prompt. If we're connecting to a service name, **that** value could be provided instead, perhaps?

I could just redefine the services as `alias_dsn`, but we use other tools that use the standard pg_service file, so I really don't want to define them twice.

从 PGCli 入口点开始，跟踪 prompt_dsn 的提供和渲染方式，然后检查连接选项 service=svc_name 的处理。完成的标准是：使用 pg_service.conf 中的服务启动的连接会在提示符中显示 svc_name，同时现有的提示符行为保持不变。

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