<!-- Provide a general summary of your changes in the Title above -->

## Description
This PR adds `use_param` and `param` to the signature configuration, in order to be able to pass the signature as a query parameter (I would appreciate some help to know how to correctly document these new fields). These parameters behave very similar to those regarding the token.

## Related Issue

#3511

## Motivation and Context

bde50bc added support for a Mashery-style signature validation, but it only supports getting the signature from a header. Mashery supports passing the signature in a query parameter, so it would be great to support this feature and mimic this behaviour.

## How This Has Been Tested

I have to say I'm super new to the Golang community so I tested this only with the test added to this PR, so I would appreciate some feedback while I build the Docker image and test it inside Kubernetes.

## Screenshots (if appropriate)

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] Refactoring or add test (improvements in base code or adds test coverage to functionality)

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes that apply -->
<!-- If you're unsure about any of these, don't hesitate to ask; we're here to help! -->
- [X] Make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). If pulling from your own
      fork, don't request your `master`!
- [X] Make sure you are making a pull request against the **`master` branch** (left side). Also, you should start
      *your branch* off *our latest `master`*.
- [X] My change requires a change to the documentation.
  - [X] If you've changed APIs, describe what needs to be updated in the documentation.
  - [ ] If new config option added, ensure that it can be set via ENV variable
- [ ] I have updated the documentation accordingly.
- [ ] Modules and vendor dependencies have been updated; run `go mod tidy && go mod vendor`
- [ ] When updating library version must provide reason/explanation for this update.
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.
- [X] Check your code additions will not fail linting checks:
  - [X] `go fmt -s`
  - [X] `go vet`

Co-authored-by: Geofrey Ernest <geofreyernest@live.com>

Fixes retrieving single policy