Add zoekt node services

What does this MR do and why?

Allows storing information about what services are enabled for a zoekt node (can be an array of services - expected values are zoekt, knowledge_graph).

We store this information in metadata and take it into account when scheduling tasks for the node (!192539 (diffs)).

Setting of this metadata is intentionally not exposed in API yet. We will decide later if we want to set list of services on node side or do some automatic categorization on Rails side. Update: we will decide later how is this setting set (#548477)

The reason for reporting which services are supported by the zoekt-indexer is that we plan to support also knowledge_graph tasks by zoekt-indexer.

References

Related to #540786 (closed)

How to set up and validate locally

This attribute can be now set from Rails console:

::Search::Zoekt::Node.first.update!(services: [0,1])

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