Search SDK: Add POST support to Search and Suggest

[brjohnstmsft]

The Azure Search REST API now supports making Search and Suggest requests
using HTTP POST instead of GET. Some customers have very large queries that
won't fit on the URL query string. POST gives them the choice to put their
queries in the request body instead.

This PR changes the behavior of the Search SDK to use POST instead of GET for
Search and Suggest calls, unless the caller sets the UseHttpGetForQueries
property on ISearchIndexClient. It also splits SearchTests and SuggestTests
into separate derived classes that exercise both POST and GET variations. This
required re-recording all the mock session records.

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