Upgrade terraform addon to helm version v0.8.0

[oanasc]

Terraform controller release v0.8.0 isn't yet in the addon catalog and features within that release are very useful, like gitRef which allows us to use a git tag module.
There are also some loose ends in the readme after this PR 367 moved charts location from https://charts.kubevela.net/addons to https://kubevela.github.io/charts

I worked around this using the helm install directly for now.

[oanasc]

Realising there is more to this, core CRD workloaddefinitions.core.oam.dev needs updating to support this