Add support for custom mapping when creating a new index using `ElasticsearchDocumentStore`

[silvanocerza]

Is your feature request related to a problem? Please describe. This is not related a problem, it's an additional functionality.

Describe the solution you'd like

I'd like to use a custom mapping when using ElasticsearchDocumentStore and creating a new index.

Describe alternatives you've considered

N/A

Additional context

This stems from this discussion.

As of now mappings are hardcoded here.

[AnushreeBannadabhavi]

I'd like to take it up if no one is working on it!

[silvanocerza]

Cool! @AnushreeBannadabhavi feel free to open a PR. :)