Add config parameter to control if compression is accepted in API responses

On fast networks, it's preferable to turn compression off with `Accept-Encoding: identity` header on all requests to SAPI.

Accepting `gzip` encoding (current default) will slow-down encoding/transmission on SAPI side, and slow down the overall download by factor of 2 to 5. This is most prominent during "solver definition fetch" while selecting solver by features, because then we must download definitions of all solvers (order of MiB).

On slow networks, it's better to leave the server-side compression.

Note: this can already be done through `DWAVE_API_HEADERS` env var and `headers` config option/kwarg, but the exact HTTP header has to be specified.