[!!!][FEATURE] Refactor TER-download handling

This change is cleaning up a lot of Extension-Manager-code under-the-hood
in order to solve some important topics related to t3x files, and how
to fetch extensions from TER / Repositories.

First of all, instead of having *one* API on how to retrieve extensions,
an ExtensionDownloaderRemoteInterface is added which allows for downloading
a single extension from a remote endpoint.

In conjunction ListableRemoteInterface is used to fetch all available
extensions from a remote (known as extensions.xml.gz currently).

For both functionalities, the existing functionality from TER API is kept:
* Downloading an extensions.xml.gz
* Downloading an extension as a t3x file from a remote

However, this is now placed into the TerExtensionRemote class.

Custom remote configuration can be added in Services.yaml.

This makes the previous "Repository" and its connected "Mirrors" functionality
completely obsolete.

Additional bugfixes:
* Since TER does not have mirrors anymore, ...