Title: Routing based on HTTP query string

Motivation:
For some applications, the HTTP query string carries information useful for routing. The prefix, path, and regex fields in the Route configuration structure only match on the part of the :path prior to the ?.

Proposal:
Add one new field to the Route configuration structure:

query_regex
(optional, string) Indicates that, in addition to prefix, path, or regex, the route additionally requires a regular expression match on the query string. The entire query string (minus the leading ?) must match the regex.

Notes

• If there's interest in this, I am willing to contribute an implementation.

Alternatives
Another way to achieve a similar result would be to support a structured config instead of a regular expression; e.g.,

"query_matches": [
  {
    "key": "env",
    "value": "test"
  },
  {
    "key": "user_id",
    "regex": "\d+[02468]"
  }
]

This approach would be more expressive than a single regex, but it would add more complexity to the HTTP connection manager filter.