[ILM] Update index management to show index lifecycle phase

[chrisronline]

Now that we can assign lifecycle policies to index templates (and therefore indices), we need a way to show the status of each index in terms of which phase of the lifecycle it is in. Index management UI is a great choice here and we need to add a column to the table that indicates (assuming the index has a lifecycle applied) which phase the lifecycle is in (we should also handle error states reported by ES here too).

This ticket talks about an explain API that will be used for this work.

[cjcenizal]

Due to confusion regarding the meaning of the phase surfaced by ES, we've removed references to it from the UI (#45484). I'm going to close this until the ES API is changed to provide a more intuitive status (elastic/elasticsearch#46357).