?fields should support fully-qualifying

### Use case

Using `?fields` with joins doesn't work if the same column name (e.g., `id`) occurs in multiple tables, because `?fields` expands to non-qualified names, so the query fails with `ambiguous identifier 'id'`

```rust
client.query("SELECT ?fields FROM table1 JOIN table2 ON table1.foo = table2.bar")
.fetch_all()
.await?;

### Describe the solution you'd like

some kind of API to tell clickhouse how to fully-qualify `?fields`. Maybe something like

```
client.query("SELECT ?fields ...")
.with_fields_qualified_by("table1")
.fetch_all()
```

### Describe the alternatives you've considered

For now I'm just writing out the fields by hand by reading `Self::COLUMN_NAMES` in a loop, but because none of the clickhouse escaping functions are public this is kind of unsafe and dumb

### Additional context

Start at the implementation of the `?fields` expansion and the query-builder API, then inspect how `Self::COLUMN_NAMES` and ClickHouse escaping are currently handled. Define the qualification API and verify it produces valid queries when joined tables share column names, without requiring callers to write fields manually.

Written by the indexing model from the issue text.