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 Extend variables support in templates used by the init command renku-python#2034
• renku project template: update the templates Change variables in the manifest file from dictionaries to objects #109
• renku ui: show the proper elements for the different types and use default values Support object variables from the template manifest renku-ui#1320