[PowerShell] Add PowerShell API client generator (WIP) (#5789)

* add powershell generator (wip)

* minor fix to api template

* rename model files

* Powershell generator fix (#11)

* Fix typo

pacakge -> package

* Add missing `petstore` subfolder to $ClientPath

* Resolve $ClientPath to absolute path

Start-Process needs WorkingDirectory to be absolute

* Fix spaces in variable name

${ somevar } is a vairable that literally has spaces in name. We need to
temporarily redifine mustache delimiters so we can generate var. names
without spaces.

* Fix typo

Remove stray `r`

* Fix *.ps1 import in module

Directory structure has changed + we should export functions using
manifest.

* Remove erroneous file

* various fixes and enhancements

* remove nullable for string

* change function name based on feedback by beatcracker

* set index to start at 0

* fix file type

* Powershell generator fix 1 (#12)

* Add closing curly brace

* Fix duplicated '['

* Get FunctionsToExport using AST

Discussion: swagger-api/swagger-codegen#5789

* add guid option to powershell generator

* add test files, remove docs

* fix array of items

* clean up powershell comment, update model/api test files

samples/client/petstore/powershell/docs/Order.md

@@ -0,0 +1,14 @@
+# IO.Swagger.IO.Swagger\Model.Order
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**id** | **Int64** |  | [optional] [default to null]
+**petId** | **Int64** |  | [optional] [default to null]
+**quantity** | **Int32** |  | [optional] [default to null]
+**shipDate** | **System.DateTime** |  | [optional] [default to null]
+**status** | **String** | Order Status | [optional] [default to null]
+**complete** | **Boolean** |  | [optional] [default to null]
+
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+