Declarative ui: convert file to a different format like .docx->.md

[tobiasKaminsky]

Related to nextcloud/files-clients#86 (comment)

With Declarative UI any server app can add interactions dynamically to Files clients.
This reduces code on client side and gives lots of opportunities to display simple stuff in a native way or interact in an easy way.

To have a starting set, we discussed and defined some actions.
One action is to offer an menu entry in context menu to convert a docx file to md.
(We can add later more).

To do so:

• add info to capability like https://github.com/tobiasKaminsky/declarativetest/blob/main/lib/Capabilities.php#L29
• create an endpoint that can receive a fileID and create a MD file out of it

[elzody]

Being implemented in the Pandoc app, most likely: https://github.com/mejo-/pandoc/pull/302