WP-CLI subcommand for retrieving current ES mapping

[tfrommen]

Is your enhancement related to a problem? Please describe.
Using the wp elasticpress WP-CLI command, I am able to create and retrieve any or all indexes. However, I can put mappings only, but not retrieve.

Describe the solution you'd like
Add a new subcommand to do just that.

Describe alternatives you've considered
If I have access to WP-CLI, but don't have this subcommand, I could do the following:

wp shell

wp> wp_remote_retrieve_body( ElasticPress\Elasticsearch::factory()->remote_request( '[<index>/]_mapping' ) );

That is unneccesarily complicated.

Additional context

[tfrommen]

This is addressed in #2378.

[felipeelia]

Closed by #2414