Introduce Code Semantic Search API

What does this MR do and why?

In this issue, we will expose a Codebase Semantic Search API in Rails. This should make use of the same ActiveContext query class currently used by Classic Chat's Codebase Search Tool executor.

Add a new GraphQL API to expose the codebase query.

• This should have the following parameters:
  • user
  • search_term
  • [{ project_ids, path (optional) }] (array)
  • knn - for the KNN_COUNT
  • limit - for the SEARCH_RESULTS_LIMIT
• This should return an array of search results by the given project_id|path combination

References

ff: #568359 Related to #555843 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

First follow the example to set up ES: !201128 (comment 2686596227)

enable the feature flag

[5] pry(main)> Feature.disabled?(:code_snippet_search_graphqlapi)
=> false

then query the graphql like:

curl -X GET "http://localhost:9200/gitlab_active_context_code/_search?pretty" -H 'Content-Type: application/json' -d '{
    "query": { "match_all": {} },
    "size": 1000,
    "track_total_hits": true
  }'

MR acceptance checklist

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Related to #555843 (closed)