* Support server-side Terraform as cloud resource provider

Provide another schematic `Terraform` to provision cloud
resource.

Fix #698

* make namespace optional
add unit-tests

* support vela show to generate componentdefinition usage

* add unit-tests

* tidy go mod

* clean go mod and updated go sum

* Remove `variable` from `properties` of Terraform compoent

* don't literally set component catagory

* refine error message when a capability could not be found

* fix ut

* Code refactor and add unittests

* fix ci issues

* Update references/plugins/references.go
Co-authored-by: Jianbo Sun <wonderflow.sun@gmail.com>

* add test cases

* fix ci issue

* revert lowercase of "Properties"

* fix ci issue

* add ut for appfile

* fix e2e issue

* add docs

* fix official site build
Co-authored-by: Jianbo Sun <wonderflow.sun@gmail.com>