Feature Request - Ability to Bind Parameter Filters to Specific Request Stages

[GuyPaddock]

As touched on briefly in #134, if a parameter has a filter defined, Guzzle Services applies that filter at the following stages:

1. Before validation, in \GuzzleHttp\Command\Guzzle\Handler\ValidatedDescriptionHandler::__invoke().
2. Before writing the value out to the wire in a request, in:
   a. GuzzleHttp\Command\Guzzle\RequestLocation\AbstractLocation::prepareValue()
   b. GuzzleHttp\Command\Guzzle\RequestLocation\AbstractLocation::resolveRecursively()
   c. GuzzleHttp\Command\Guzzle\Serializer
   d. ...and several different request location handlers.
3. After reading a value in from a response on the wire, in several response location handlers.

This leads to the following awkwardness:

• If a filter like json_encode() is being used to control the way a parameter is being written-out to the wire:
  • It inadvertently has an effect on the format of the parameter on its way through validation. So, for example, a value that should come in as an array has to be declared as a string to prevent validation from erroring-out on the JSON string. There's no way to make these types of filters happen only after validation.
  • It gets applied twice (in both 1 and 2 above), so instead of sending a JSON string, it sends a JSON-escaped JSON string (i.e. the value gets double-encoded, as a quoted JavaScript string instead of JSON).
• Default values only run through one filter (2), which leads to different results than if you passed-in a value equal to the default. IMHO, the default value should be run through validation the same way as passed-in values, but that's a separate issue.
• Filter functions/methods don't really have a good way to differentiate between filtering that happens to values going out in a request and values coming in from a response. This is likely not a huge issue since different parameters are used for request and response data, AFAIK.

Ideally, there'd be a way to "bind" a filter to a particular stage of the process, so that it's only invoked when desired.