Added Elasticsearch::get_documents and Indexable::multi_get

[felipeelia]

Description of the Change

This is a "take over" of #1959

We needed an easy way to fetch multiple documents per API call, so we added get_documents to the Elasticsearch class and then added multi_get to the Indexable class for ease of use.

Props @nickdaugherty
cc: @rinatkhaziev @pschoffer @netsuso @parkcityj

Alternate Designs

N/A

Benefits

Allows you to get multiple documents per API call.

Possible Drawbacks

N/A

Verification Process

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

Changelog Entry

Added Elasticsearch::get_documents and Indexable::multi_get. Props @nickdaugherty and @felipeelia

[Rahmon]

LGTM 👍

[felipeelia]

@oscarssanchez as all tests have passed, can you please give this a final review and merge it? Thanks!