Add posibility to import PlantUML diagrams

[oanalavinia]

draw.io supports importing PlantUML diagrams, see https://www.diagrams.net/blog/import-formats
It would be nice to be able to enable this feature for Diagram Application.

[oanalavinia]

Similar to PDF export, there is an external server used by draw.io for importing PlantUML diagrams: https://plant-aws.diagrams.net.
In this case, a configuration can be added to enable the PlantUML import with the possibility to provide a custom server, if the user doesn't want to expose content to an external service.

Information for setting up your own server can be found at https://github.com/jgraph/plantuml-server and https://github.com/jgraph/plantuml-serverless.
Note that if disableExternalServices is enabled, see #105, this feature will also be impacted.

Note also that any PlantUML server might work, like http://www.plantuml.com/plantuml or https://plantuml.nitorio.us. After testing them, you can use their documentation to deploy your own.