Opensearch: Replace error source http client with a new error source methods

[ivanahuckova]

We want to deprecate and get rid of errorsource/httpclient as a part of https://github.com/grafana/grafana-plugin-sdk-go/issues/1105, as error source middleware implemented in experimental error source httpclient is turning any 400+ response to error and hiding original message.

Instead of that we have introduced 2 methods developers can use:

• backend.ErrorSourceFromHTTPStatus to get error source from http code
• backend.IsDownstreamHTTPError to get error source from http error

In this PR we are removing errorsource/httpclient and instead just using those methods.

[ivanahuckova]

Thank you @iwysiu for review! I have fixed the ppl part: https://github.com/grafana/opensearch-datasource/pull/505/commits/403a246380a5582e2c2d23742b3540df45b1d498