Add language filter support for blob searches

This is step 3 of the Add search filtering by programming language epic (full plan is detailed in the epic).

Blob searches will need to support accepting filtering for languages. This should be implemented similar to other facets (example: issues, merge requests) with the addition of allowing multiple languages to be selected and sent to Elasticsearch.

High level plan:

• add the new param to the list of allowed params (could be some refactoring here as I don't see other filters added to the allowed params in SearchHelper). We should only allow this filter param for blob searches
• a new Language filter similar to Elastic::Latest::StateFilter Not required
• apply the new filter in the search_blob method found in Elastic::Latest::GitClassProxy