templatise plug-ins/add-ins in project creation

[gavin-k-lee]

In the context of the VNC and other targeted use-cases, it would be nice to templatise/modularise the various options, for example like:

To do:

• scope out necessary functionality used in jinja
• see what changes in renku-python are needed, if any
• put together a PoC

[gavin-k-lee]

Changes required:

• renku python/core: enforce the new variables metadata https://github.com/SwissDataScienceCenter/renku-python/issues/2034
• renku project template: update the templates https://github.com/SwissDataScienceCenter/renku-project-template/issues/109
• renku ui: show the proper elements for the different types and use default values https://github.com/SwissDataScienceCenter/renku-ui/issues/1320