Optimize job work to index provider data in elastic

[mmikirtumov]

IKnowledgeProvider gives us an opportunity to optimize job works. Now we can move the logic of adding the indexes for providers (jira, confluence, question - answer) into core and in scheduler enable or disable specific provider.
From code part I will suggest to use the approach https://www.stevejgordon.co.uk/asp-net-core-dependency-injection-registering-multiple-implementations-interface.
We can just create extension method like EnableProvider() and register provider using DI container.