Add sub-extension to resource files in built-in templates

[shreyas-goenka]

Changes

We want to encourage a pattern of only specifying a single resource in a YAML file when an .<resource-type>.yml (like .job.yml) is used. This convention could allow us to bijectively map a resource YAML file to it's corresponding resource in the Databricks workspace.

This PR simply makes the built-in templates compliant to this format.

Tests

Existing tests.

[pietern]

@shreyas-goenka Can you update the PR summary?

Change LGTM.

[shreyas-goenka]

@pietern I have updated the PR description.