HTTP body application/x-www-form-urlencoded support

[owen-d]

Describe the bug
According to https://prometheus.io/docs/prometheus/latest/querying/api/, many endpoints allow specifying query string parameters in the body via the application/x-www-form-urlencoded header. In an effort to maintain parity, we should support this on the following endpoints:

• POST /api/v1/query_range
• POST /api/v1/query
• POST /api/v1/series

This can be easily handled in https://github.com/grafana/loki/tree/master/pkg/loghttp