oneclickexport: add external_services table DB query processor

Created by: sashaostrikov

This commit introduces a new type of processors, which perform DB queries to arbitrary tables of frontend database.

DB queries are grouped in one array of ExportRequest as an alternative to previous "each export part is a separate JSON property" approach.

Test plan: new unit test is added. When the REST endpoint will be in place, manual tests will also be performed.

Depends on https://github.com/sourcegraph/sourcegraph/pull/39855

Closes https://github.com/sourcegraph/sourcegraph/issues/39620