Add connection setting to enable/disable HTTP pipelining

Previously, pipelining was explicitly disabled since elasticsearch did not support it.  Support has just been added (https://github.com/elasticsearch/elasticsearch/pull/8299) and so this PR adds a connection setting to enable/disable pipelining on the client as well.

Pipelining can be turned on at a global/connection level via `IConnectionConfigurationValues` or per request via `IRequestConfiguration`.  The default is to disable pipelining (might make sense to default to enabled in `2.0`).