Story: Partial migration of data by entity types

Currently the tool can migrate data all or nothing. But users are often willing to migrate only orders or only customers or catalog plus customers etc. See #497, #489, #337, #427, #326, #406, #80

Acceptance Criteria:

- ability to migrate data separately by its entity type:
-- customers
-- orders
-- catalog
-- CMS
-- rules
-- all other
- Partial migration of entities. For example: only last 1000 orders or orders were created for a specific period of date/time

:warning: Since there are many interconnections between entities (customers, orders, catalog ...) partial migration can break M2 store. For example if a developer migrate only orders and customers then merchant can potentially not able to process orders without existing products. This needs to be somehow accounted for not to introduce database inconsistency.

Start with this issue and the related issues #497, #489, #337, #427, #326, #406, and #80. Define how entity-type and date-range selection should work, then determine how partial migrations can preserve consistency between customers, orders, catalog, CMS, rules, and other entities; done means the scope and safety behavior are specified and verified.

Written by the indexing model from the issue text.