fix: Loki now works with username and password in the config

[chasers]

• Loki backend now takes the username and password from the backend config and encodes it for basic auth
• updated and added some tests

## PATCH backend
curl -X "PATCH" "http://localhost:4000/api/backends/cafe18a6-505c-4412-b838-be2aab7ee507" \
     -H 'Authorization: Bearer 6404d476fd62450e6f76497b9eabcc7528e914b69aebb17507393c38560f6a62' \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{
  "metadata": {},
  "config": {
    "username": "me",
    "password": "god",
    "url": "https://logs-prod3.grafana.net/loki/api/v1/push"
  },
  "name": "loki-test",
  "type": "loki"
}'