feature: column aliases in cat indices API

[larrycinnabar]

Feature: allow column (?h=) aliases in cat indices API

Why this should be part of the repo?

This repository allows to call any elastic request, so if elastic can execute http://localhost:9200/_cat/indices?h=idx, then this repo should too. (having idx instead of index)

Implementation

• in cat_indices.go, when url is built - given columns are looked up in the alias map and replaced if any
• an alias can be given to multiple original fields because of backwards compatibility (depending on which of 7.* version, you may have refresh.time or refresh.external_time, and they both have rti alias)
• alias map is constructed on the init stage (using init() function). It uses reflect, but as it's one-time on-startup call, I consider this not harmful.
  Other option (to avoid reflect) is to manually declare the alias map. But this approach has a drawback as well: we declare fields in two places manually: the CatIndicesResponseRow struct and the aliases map. And when updating further, developer is required to update BOTH things, so it will become a danger place that can admit bugs when developer updates one thing and forgets about other.
• Tests are added:
  • very simple test to check if alias map exists
  • test on ?h=h to be the same as ?h=health
  • test on double alias with ?h=rti

[larrycinnabar]

@olivere any ideas, arguments, suggestions?

[olivere]

Just a comment to simplify things a bit.

[larrycinnabar]

Upd: switched from alias reflect-generation to manual hard-coded map

Ready for merging

[olivere]

Thanks. Will be released in 7.0.10.