Create a version of `Identifier` that handles qualified identifiers

### Use case

We often want to refer to qualified identifiers (e.g., `table.column`). Right now, the `Identifier` type only handles a single level of identifiers. As far as I know, Clickhouse only supports two levels of nesting (there are no schemas or other objects that would require quoting `foo.bar.baz`); however, it is perfectly legal for an Identifier to include a `.` character.

### Describe the solution you'd like

A new type

```rust
pub struct ScopedIdentifier<'a, 'b>(pub &'a str, pub &'b str)
```

This should serialize as follows:

```rust
ScopedIdentifier('foo', 'bar') -> `foo`.`bar`
ScopedIdentifier('', 'bar') -> `bar`
ScopedIdentifier('foo', '') -> `foo`.``
```

### Describe the alternatives you've considered

Right now I'm doing this in my own query generation with `format!()` and it's bad.

### Additional context

Search the existing `Identifier` type and its serialization implementation, then inspect nearby tests or query-generation code. Add the requested `ScopedIdentifier` behavior for qualified and empty components, and verify the serialized forms match the examples in the issue.

Written by the indexing model from the issue text.