Add graph DB connection management

What does this MR do and why?

Adds graph DB connection management:

• allows opening new connections to kuzu DBs
• keeps connection pool of existing DB connections
• closes unused DB connections
• adds query endpoint to webservice (querying itself will be added in a separate MR)

Related to #81 (closed)

How to set up and validate locally

• build zoekt-indexer with make build-unified and restart gitlab-zoekt-webserver-development-* gdk services
• on Rails side checkout gitlab!199424 (closed) and use rails console to execute some queries (this example assumes that namespace_id=34 already has kuzu DB, or you can just copy manually any kuzu DB to the expected file location):

[1] pry(main)> ::Gitlab::Ai::KnowledgeGraph::Client.new.query("anything", namespace_id: 34, node_id: 1)
#<HTTParty::Response:0x6dbc8 parsed_response={"error"=>"Querying not implemented."}, @response=#<Net::HTTPInternalServerError 500 Internal Server Error readbody=true>, @headers={"content-type"=>["application/json"], "date"=>["Thu, 07 Aug 2025 06:29:22 GMT"], "content-length"=>["38"], "connection"=>["close"]}>