Vespa CLI option to delete all documents in an application

[rharige]

Is your feature request related to a problem? Please describe.
Feeding or re-feeding data is a common task during the development and testing of schema changes. Often, it is desired to remove all documents from the application index to re-test the data-feeding process. Currently, there is no straightforward (or convenient) method to delete all documents from a Vespa application index.

Describe the solution you'd like
Since the Vespa CLI has become the standard tool for interacting with Vespa applications, it would be beneficial to include a drop command that allows users to drop the application index, effectively deleting all documents associated with it. There is a destroy command in the Vespa CLI, which currently only works for Vespa Cloud deployments. While I haven't tested its behavior, it may be possible to extend its functionality to also work with local or non-cloud Vespa deployments.

Describe alternatives you've considered
There are documented options to batch-delete the documents. But it is not convenient, as you have to construct a right curl command to run on the terminal. Whereas an option to do so in Vespa CLI would be far more convenient.

Additional context
Add any other context or screenshots about the feature request here.