[HTTPCLIENT-2046] Rename AuthSchemes enum to StandardAuthScheme final class

Modeled after StandardCharsets, the new class indicates a non-exhaustive
list of standard auth schemes by name supported by the HttpClient. The
previous enum suffered from two issues:

* it was exhaustive implying that no more auth schemes can be supported
* its sole purpose was to contain an id for the declared auth scheme;
  the enum values theirselves were never used directly

This closes #196
42 files changed