add spinner and loading text while processing `install.yml`

Right now, when installing a template via `aio app add action` for example -- the command appears to "hang" when processing the `install.yml` file since we don't provide any loading text or spinner to tell the user that something is happening. This will become more apparent if the user's network is slow or the Dev Console server is taking longer than expected.

Suggestion: add some loading text and spinner before/after here: https://github.com/adobe/aio-cli-plugin-app-templates/blob/900add6b49e01234c5c280ebbd2b74c6072f4bb0/src/commands/templates/install.js#L73

Open src/commands/templates/install.js around line 73 and trace the install.yml processing triggered by the `aio app add action` entry point. Verify that loading text and a spinner appear while processing, then stop after processing completes.

Written by the indexing model from the issue text.