issue-85: enable custom headers for every requests

[dmitryk-dk]

Updated logic with settings params.
Grafana returns part of the headers in the JSONData as json.RawMessage with names headerName*
and values are stored in the settings.DecryptedSecureJSONData

So it looks like

JSONData:=map[httpHeaderName1:AccountID httpHeaderName2:ProjectID maxLines:20000]
settings.DecryptedSecureJSONData:=map[httpHeaderValue1:0 httpHeaderValue2:0]

In that case i changed the settings variable from struct to map it helps to handle multi headerNames because we can set it more than 1

Related issue: #85