Prepared Statement connection is active and wait for ClientRead.

Hi team,

Right now, asyncpg does prepared statements in two steps.
1. Parse, Describe, Flush.
2. Bind, Execute, Sync.

Two coroutines represent these two steps. In a highly concurrent setup, the wall-clock gap between these two steps can be large. Why is it causing a problem for me? Command `Flush` does not change the connection state, i.e., `active`. So I observe a lot of queries show up as
```
wait_event_type | wait_event | state
-----------------+------------+--------
Client | ClientRead | active
```
in `pg_stat_activity`. This causes a lot of unnecessary confusion for our database monitors/observability tools. The connection is essentially idle between these two steps, so I think it is better to report it as `wait_event=ClientRead` but `state=idle`? Meanwhile, the command `Sync` will change the connection state to `idle`, so after step 2, the connection becomes idle.

My ask is: could we use `Sync` instead of `Flush` in step 1?

Moreover, there could be more than one `bind` step, so the sequence could be
```
1. Parse, Describe, Flush.
2. Bind, Execute, Sync.
3. Bind, Execute, Sync.
...
```
In this case, the connection is active only between step 1 and step 2, but idle for all other step gaps. I kind of feel this behavior is inconsistent.
I might have neglected some basic design about PostgreSQL [extended query](https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY). Hope to hear from you soon!

Start with asyncpg’s prepared-statement flow described in the issue and PostgreSQL’s extended-query protocol documentation. Determine whether changing Flush/Sync sequencing can preserve correctness with multiple Bind/Execute steps; done means pg_stat_activity reports the connection state consistently without breaking prepared statements.

Written by the indexing model from the issue text.