Add support for forcemerge action in ILM policy hot phase

[cjcenizal]

Elasticsearch introduced this capability into 7.7 via elastic/elasticsearch#52073. Note that the UI should only allow this action in the hot phase if the user has enabled rollover.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[clyfish]

@cjcenizal
Any update?

[clyfish]

elastic/elasticsearch#58289
elastic/elasticsearch#56377
readonly and shrink actions are also demanded to be in hot phase.

[yuliacech]

Hi @clyfish, just to confirm: all 3 actions forcemerge, readonly and shrink are only allowed in the hot phase, if the user enabled rollover, right?
Also, is there any dependency between readonly and shrink? And if there should a specific order of these actions in UI?

[cjcenizal]

@yuliacech I think @clyfish is just pointing out that there are Elasticsearch issues tracking work to add support for readonly and shrink actions to the hot phase. Once that work is done, we can do work to support those options in the UI too, but I would consider that out of scope for this issue. For now, I think we should just implement the forcemerge action. From the docs:

To use the forcemerge action in the hot phase, the rollover action must be present. If no rollover action is configured, ILM will reject the policy.