Allow request parameters to modify datasource endpoint

[jimlambie]

Currently datasource filters can be modified when the request contains parameters that match with a datasource's requestParams object. For example:

"requestParams": [
  {
    "param": "channel",
    "field": "channel"
  }
]

When a route such as /channels/:channel is requested, the :channel parameter is added to the request. e.g. /channels/three will result in req.params = { "channel": three" }. This is added to a datasource filter such that ?filter={"channel":"three"}

There is now a requirement to be able to modify a datasource endpoint in a similar way, for example it'd be ideal to inject parameters into the endpoint from the request, when endpoints like 1.0/channels/{channel} or 1.0/channels?channel={channel} have been specified.