[7.x] Add is_write_index column to cat.aliases

[talevy]

Aliases have had the option to set is_write_index since 6.4,
but the cat.aliases action was never updated.

example:

alias            index                              filter routing.index routing.search  is_write_index
filebeat-7.0.1   filebeat-7.0.1-2019.07.23-000011   -      -             -               -
metricbeat-7.0.1 metricbeat-7.0.1-2019.07.23-000005 -      -             -               true
metricbeat-7.0.1 metricbeat-7.0.1-2019.07.21-000003 -      -             -               false

backport of #44772