Add compression support

[jobergum]

Adding accept-encoding to the request will make Vespa compress the response, which can reduce latency for cross-region query traffic:

Accept-Encoding: gzip

[thomasht86]

This is the default behavior for both httpx and requests.
See
They will also be decompressed automatically.
See #883

• From httpx docs:

Any gzip and deflate HTTP response encodings will automatically be decoded for you.

• From requests FAQ:

Requests automatically decompresses gzip-encoded responses, and does its best to decode response content to unicode when possible.