RFC 201: Update auto indexer execution

Created by: efritz

This is part of RFC 201: Auto-indexer build configuration.

Convert the indexing process from being hard-coded to being parameterized by the types defined in https://github.com/sourcegraph/sourcegraph/issues/13892.

For the time being we will keep the effective parameters the same by only executing one instance of a configured task, which is hard-coded to behave as it does today.