Integrate with Meilisearch search engine

[regisb]

As part of the transition from Elasticsearch to Meilisearch, we need to make sure that the forum works with this new search engine. Thus, we need to implement a togglable search engine which will query Meilisearch instead of Elasticsearch, both for storage and retrieval.

We need to implement the following:

1. Create a FORUM_SEARCH_ENGINE setting, which will point to a search engine class imported at runtime.
2. Create a MeilisearchBackend class that exposes a similar API to the ElasticsearchBackend. Make sure that both classes inherit from an abstract BaseSearchBackend class. Replace all instances of ElasticsearchBackend in the codebase by BaseSearchBackend.
3. Implement the required methods of MeilisearchBackend class. Do this in a test-driven development (TDD) manner.

Prior work exist which we can use as reference:

• Student notes search backend: feat: meilisearch backend for notes search edx-notes-api#444
• edx-platform search engine: feat: add Meilisearch-compatible search engine with tests edx-search#164

[regisb]

Closed by #126