Several adjustments needed

[daninden]

Allow extension of models (remove sealed):

• ContentItemCreateModel
• AssetWithRenditionsReference

Allow settable Id (for unittests):

• ElementMetadataBase

Return null for content not found instead of an exception:

• ActionInvoker
• ManagementHttpClient

Allow injection of HttpClient

• ManagementClient
• HttpClient

Motivation

Fixes #270
And several changes that allow us to extend the models used, and to not throw Exceptions on items not found (but rather return null instead). This is a breaking change ofcourse, but having to catch Exceptions to check if something exists is very cumbersome.

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?