dvc machine: implement initial terraform support (#6462)

* tpi: add naive terraform-provider-iterative wrapper

* executor: add naive backend/terraform wrapper

* setup: add dvc[terraform] install extras

* config: add feature.executor feature flag

* fix named executor config sections

* executor: add poc add/modify/remove/init/destroy commands

* relax python-terraform requirement

* ci: install terraform extras

* tpi: support .tf.json format

* s/executor/machine

* terraform: use separate dirs per machine

* s/init/create/

* reserve local machine name

* add initial ssh skeleton

* add initial machine unit tests

* add initial machine command tests

* add tests for add/remove

* enable feature by default in test env

* close temp pemfile object so it can be used on windows

* cleanup pemfile syntax

.github/workflows/tests.yaml

@@ -27,7 +27,7 @@ jobs:
     - name: Install requirements
       run: |
         pip install wheel
-        pip install ".[all,tests]" Pygments collective.checkdocs pre-commit
+        pip install ".[all,tests,terraform]" Pygments collective.checkdocs pre-commit
     - name: install temporary dependencies
       run: |
         pip install git+https://github.com/isidentical/gdrivefs@service-account
@@ -77,7 +77,7 @@ jobs:
       if: steps.cache.pip-cache-dir.cache-hit != 'true'
       run: |
         pip install --upgrade pip setuptools wheel
-        pip install -e ".[all,tests]"
+        pip install -e ".[all,tests,terraform]"
     - name: install temporary dependencies
       run: |
         pip install git+https://github.com/isidentical/gdrivefs@service-account