Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata. This new endpoint replaces the /{index}/_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers and /_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers endpoints whilst maintaining the same response formats. The only exception to this is on the /_alias|_aliases|_warmer|_warmers endpoint which will now return a section for 'aliases' or 'warmers' even if no aliases or warmers exist. This backwards compatibility change is documented in the reference docs.

Closes #4069