Add external plugins on install

[AlexanderSkrock]

Is your feature request related to a problem? Please describe.

Currently it is only possible to link a locally available plugin which is great for developing a plugin. But in case the modeler is used for e. g. modelling other plugins such as the transaction boundary plugin could be great.

Describe the solution you'd like

It would be great if there was a second option aside of "--link-plugin" called "--copy-plugin" which will add the required plugin to the installation.

We should automatically detect whether we have a local path or external url. For external urls we need to download first. It would be great if plugins would be cached as well.
Then, we need to check whether it is a directory, then copy directly or an archive, then extract to the plugins directory.
To make it clearer "--link-plugin" should write an error message when an external url was given, saying that "--copy-plugin" should be used.

Describe alternatives you've considered

There is none except manually downloading and then use the linking functionality but that is both inconvenient and not what we expect, because linking is not necessary.