When a request going through Grafana data source proxy responds with a websocket
upgrade response we cannot override the response body since it produces an error.
This problem seems to have been introduced in Grafana v8.0 by #38962.
In addition #40303 added same problem.
Fixes#41292
* Introduce response_limit for datasource responses
* Fix lint
* Fix tests
* Add case where limit <= 0 - added parametrized tests
* Add max_bytes_reader.go
* Use new httpclient.MaxBytesReader instead of net/http one
* Fixes according to reviewer's comments
* Add tests for max_bytes_reader
* Add small piece in configuration.md
* Further fixes according to reviewer's comments
* Fix linting - fix test
