Add support for `*.tfvars` files in template resources

[ethanndickson]

Once coder/coder#13984 is cherry-picked into the next release, the provider can use codersdk to parse *.tfvars files when creating template versions.
We can't use a go mod replace in the meantime, as there's breaking codersdk changes in main where the respective coderd changes aren't in the docker image we use for tests.