✨ : use a tfvar file in the runner instead of sending variables as command line parameters

[juwit]

Is your feature request related to a problem? Please describe.
As of today, the runner uses command line parameters to send variables to terraform.
We should generate & use a tfvar file instead

Describe the solution you'd like
Expose a route for the runner to download the tfvar file.
Add a curl call in the runner to download the tfvar file before running terraform commands.

Describe alternatives you've considered
Generating the tfvar file directly in the runner script, kind of this way :

echo '[gaia] generating tfvars variable file'
echo '{{{ tfvars }}}' > gaia.auto.tfvars

This should work, but may break if a variable string value contains a '.

Additional context
N/A