* `QDRANT_HOST` variable is used by all openapi tests, removed its copies from
`test_multi_vector_uint8.py`, `test_multi_vector_unnamed.py`, `test_query.py`,
`test_shard_snapshot.py`, and `test_snapshot.py`
* Added optional `QDRANT_HOST_HEADERS` env variable to set custom HTTP headers
in order to reach Qdrant instance behind a reverse proxy. The content of the
variable is JSON payload, e.g. `{"host": "qdrant.local"}`
* Adapted `./tests/basic_*.sh` scripts to aware of `QDRANT_HOST_HEADERS` env variable
