Add legacy option for run_as

[shreyas-goenka]

Changes

This PR partially reverts the changes in #1233 and puts the old code under an "experimental.use_legacy_run_as" configuration. This gives customers who ran into the breaking change made in the PR a way out.

experimental:
  use_legacy_run_as: true

Tests

Both manually and via unit tests.

Manually verified that run_as works for pipelines now. And if a user wants to use the feature they need to be both a Metastore and a workspace admin.

---

Error when the deploying user is a workspace admin but not a metastore admin:

Error: terraform apply: exit status 1

Error: cannot update permissions: User is not a metastore admin for Metastore 'deco-uc-prod-aws-us-east-1'.

  with databricks_permissions.pipeline_foo,
  on bundle.tf.json line 23, in resource.databricks_permissions.pipeline_foo:
  23:       }

---

Output of bundle validate:

➜  bundle-playground git:(master) ✗ cli bundle validate
Warning: You are using the legacy mode of run_as. The support for this mode is experimental and might be removed in a future release of the CLI. In order to run the DLT pipelines in your DAB as the run_as user this mode changes the owners of the pipelines to the run_as identity, which requires the user deploying the bundle to be a workspace admin, and also a Metastore admin if the pipeline target is in UC.
  at experimental.use_legacy_run_as
  in databricks.yml:13:22

Name: bundle-playground
Target: default
Workspace:
  Host: https://dbc-a39a1eb1-ef95.cloud.databricks.com
  User: shreyas.goenka@databricks.com
  Path: /Users/shreyas.goenka@databricks.com/.bundle/bundle-playground/default

Found 1 warning