Support multivariate feature flags, and more!

Added

• Support for multivariate feature flags. New methods boolVariation, jsonVariation and intVariation and doubleVariation for multivariates.
• Added LDClientInterface, an interface suitable for mocking LDClient.

Changed

• The Feature data model has been replaced with FeatureFlag. FeatureFlag is not generic.
• The allFlags method now returns a Map<String, JsonElement> to support multivariate flags.

Deprecated

• The toggle call has been deprecated in favor of boolVariation.

Removed

• The getFlag call has been removed.
• The debugStreaming configuration option has been removed.