GET _indices API

[rashidkpc]

With the removal of _aliases (#4539) we've lost an endpoint for efficiently listing available indices & aliases, which Kibana relies on for computing timestamped indices.

It would be nice to have an _indices endpoint that could efficiently return a list of indices with a configurable amount of additional granularity, eg mappings, settings, aliases.

Bonus points if we could support wildcard matching here for both index and alias names, eg

** Every index name **

/_indices

** Every index with its aliases and settings **

/_indices?aliases

** Every index named mydata-* or with an alias matching mydata-, along with its mapping *

/mydata-*/_indices?mapping

[clintongormley]

Closing in favor of #4069