enhance the helm chart with the ability to set custom plugins based on config-file

[nirrozenbaum]

we should update the helm chart to have the ability to inject custom plugins, possibly using both internal plugins (in tree plugins) as well as plugins from outside of the repo (out of tree plugins).
this could be mirrored from epp helm chart, where we allow the use of custom plugins in a config map as a mounted volume.

a pre-req is to first complete the plugins config loader

[nirrozenbaum]

depends on #77