`Invoke-RestMethod` & `Invoke-WebRequest` should have tab completion for `-ContentType`

Summary of the new feature / enhancement

As a user who tests API REST calls from the terminal using Invoke-RestMethod & Invoke-WebRequest frequently, it would be very beneficial to have tab completion for content types. Especially for common ones like application/json or application/x-www-form-urlencoded which can get cumbersome to type out every time.

Given the list of MIME Types is quite big and most of these are not commonly used in PowerShell, we could limit tab completion to common content types.

Quick grep search of content types across GitHub repos for PowerShell:

• application/json
• application/x-www-form-urlencoded
• application/xml
• application/zip
• application/octet-stream

We could even use .NET Media MIME types in System.Net.Mime.MediaTypeNames to load these in dynamically and avoid hardcoding a list.

It would improve the developer experience of these cmdlets, especially when testing API calls.

Proposed technical implementation details (optional)

• Add a ContentArgumentCompleter to virtual property ContentType in WebRequestPSCmdlet base class.
• Ensure user can do something like Invoke-RestMethod -ContentType <TAB> and cycle through completions of content types.
• If they provide start of word it would complete the possible content type completions e.g. Invoke-RestMethod -ContentType 'app<TAB> would complete to Invoke-RestMethod -ContentType 'application/json'.

Start with the WebRequestPSCmdlet base class and its virtual ContentType property, then review how argument completers are implemented for PowerShell cmdlets. Add completion for common content types for both Invoke-RestMethod and Invoke-WebRequest, including prefix matching such as “app”, and verify that cycling through completions works.