Query String Policy: Manipulate request URI query string

Manipulate request URI query string by adding extra parameters and/or make use of named arguments for transformations. It should offer similar interface to the Headers policy
Use cases:

• Merge params in the request and in the redirect URL e.g.
  Pattern: /test
  Redirect: <host>/internal/api/test?param1=extraparam
  Actual request: /test?foo=bar
  Actual redirect: <host>/internal/api/test?param1=extraparam&foo=bar
  If they are both in the request and in the redirect URL, keep the ones in the destination redirect URL
• Map URL /{orderId}/{accountId}/{incFields} into /sales-order/svc/v2/testDoc/{orderId}?account={accountId}&incFields={incFields}. This should work when request URL contains account or incFields parameters in the query string.

This feature is closely related with the URL rewriting policy so it's under consideration whether implementing this in a separate policy or not (see related GH issue)

GH issue: https://github.com/3scale/apicast/issues/711