Add get-mapping WP-CLI Subcommand

[felipeelia]

This is merely a takeover of #2378.

Description of the Change

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.

This PR adds a new subcommand, wp elasticpress get-mapping, that allows for retrieving the current mappings:

wp elasticpress get-mapping

By passing an optional index name, the command will return the mapping for that index only:

wp elasticpress get-mapping --index-name=ep-examplecom-post-1

Alternate Designs

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.

Benefits

More complete API/feature set. Easier to use than wp shell command.

Possible Drawbacks

I don't see any.

Verification Process

I tested this locally, both with and without passing an index name.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

#2379

Changelog Entry

### Added
- New WP-CLI subcommand for retrieving mappings: `wp elasticpress get-mapping [--index-name]`. Props [@tfrommen](https://github.com/tfrommen) via [#2378](https://github.com/10up/ElasticPress/pull/2378).

[Rahmon]

I think we need to chek if it's not empty ! empty( $assoc_args['index-name'] ). Using (array) isset( $assoc_args['index-name'] ) is always returning true. PHP is converting to array the result of isset: [ 0 => false ].

Consequently, join is receiving a NULL value and emitting a Warning.

[Rahmon]

When we pass an index name (string), we shouldn't call join because it expects an array.

[felipeelia]

Good catch @Rahmon, do you mind giving another look?