Expose Conn.preparedStatements to TraceQueryStart

**Is your feature request related to a problem? Please describe.**
I'm currently writing a custom Prometheus tracer, and I want to expose the following:
- query
- duration
- success/failure

I tried doing so using `TraceQueryStart` / `TraceQueryEnd` following the example provided in `tracelog`. I noticed that in `TraceQueryStart`, when trying to access `data.SQL`, we only get `pgx_0`, `pgx_1`, etc. (we're using the stdlib adapter)

**Describe the solution you'd like**
I'd like [`Conn.preparedStatements`](https://github.com/jackc/pgx/blob/master/conn.go#L70) to be exposed to `TraceQueryStart`.

Either directly:

```diff
- preparedStatements map[string]*pgconn.StatementDescription
+ PreparedStatements map[string]*pgconn.StatementDescription
```

or indirectly:

```go
func (c *Conn) GetPreparedStatement(name string) *pgconn.StatementDescription {
return c.preparedStatements
}
```

**Describe alternatives you've considered**
An alternative I have in mind is to implement `TracePrepareStart` to keep a map[name]sql on my side but it's a bit annoying, and I'm not sure `name` is unique across all conns when using a pool

**Additional context**

Start with conn.go's preparedStatements and the TraceQueryStart/TraceQueryEnd hooks, then compare the stdlib adapter path with the tracelog example. Done means TraceQueryStart can associate generated prepared-statement names with their original SQL without requiring a separate per-pool map; the chosen API should be validated against the existing tracing behavior.

Written by the indexing model from the issue text.