Remove types from Get/MultiGet

[romseygeek]

This commit removes types from the ShardGetService, and propagates this API change
up through the Transport and Rest actions for Get and MultiGet

Relates to #41059

[elasticmachine]

Pinging @elastic/es-search

[jimczi]

LGTM

[alpar-t]

@elasticmachine test this please

[romseygeek]

@elasticmachine test this please

[romseygeek]

I've chased down a bunch of test failures here due to a change in MultiGetRequestBuilder that doesn't get caught by the compiler. The #add(String index, String type, String id) method is removed, but calling code that uses this signature is silently redirected to the #add(String index, String... ids) method, with the result that the old type parameter gets repurposed as a String id.

This could introduce some quite nasty and hard-to-track-down bugs into client code over an upgrade. I think the best solution is probably to rename the add() methods that take doc ids to addIds(), which will at least force a recompilation in client code