Cat API: Add endpoint to show aliases

This endpoint allows to check aliases, their indices, if a filter is
configured along with routing values for searching and indexing.

Basic usage:

```
curl "localhost:9200/_cat/alias?v"
alias                     index filter index_routing search_routing
test-alias-filter         test1 *      -             -
test-alias-search-routing test1 -      -             foo,bar
test                      test1 -      -             -
test-alias-routing        test1 -      foo           foo
test-alias-index-routing  test1 -      foo           -
test-two-indices          test1 -      -             -
test-two-indices          test2 -      -             -
```

Closes #4414