CXX-2785 do not apply readConcern or writeConcern to search index commands

[kevinAlbs]

Summary

Do not apply readConcern or writeConcern to search index commands

Tests verified with this patch: https://spruce.mongodb.com/version/654b89333e8e86f7a940c6e3

Background & Motivation

This is a follow-up to #986

Search index commands return an error if readConcern or writeConcern is included.

Prior to the fix, applying a non-default readConcern or writeConcern to the mongocxx::collection, mongocxx::database, or mongocxx::client resulted in the readConcern or writeConcern being included in the search index command.

Prior to the fix, the added test "create and list search indexes with non-default readConcern and writeConcern" fails when run against an Atlas cluster:

/Users/kevin.albertson/code/tasks/mongo-cxx-driver-CXX-2697/src/mongocxx/test/search_index_view.cpp:246: FAILED:
  REQUIRE( siv.create_one(name, definition.view()) == "test-search-index-case6" )
due to unexpected exception with message:
  Command does not support writeConcern: generic server error

These changes are intended to be included in 3.9.0. The API added in #986 has not yet been released. The error may give a poor user experience.

The new tests are specified in mongodb/specifications#1474.

[kkloberdanz]

Looks good!