Support `X-ClickHouse-Summary` header

[abonander]

The response from the server includes a X-ClickHouse-Summary header with some potentially useful information:

 {"read_rows":"0","read_bytes":"0","written_rows":"0","written_bytes":"0","total_rows_to_read":"0","elapsed_ns":"662334","memory_usage":"8451671"}

The header becomes even more useful if passing the wait_for_query_end=1 option.

Propose adding:

• Query::execute_with_summary()
• Insert::end_with_summary()

that set wait_for_query_end=1 and parse the X-ClickHouse-Summary header from the response.

Otherwise, maybe we just expose the response itself, as it's also possible to set custom headers.

[abonander]

Thinking more about this, I think we could expose a map-style API so that we don't have to fix the structure in stone, in the event that keys are added to or removed from the header over time.

Alternatively, we could just expose the raw JSON string and have the user deserialize it themselves.

But right now, there's no way to get access to it whatsoever, so it would be helpful to have something.