[ILM] Add ILM action for freezing an index

[colings86]

When frozen indices are implemented (#34352) we should add a new action to ILM so a user can freeze indices in the cold phase. The actual implementation of the action will depend on the final API for frozen indices.

[elasticmachine]

Pinging @elastic/es-core-infra

[s1monw]

@colings86 is this still stalled? Can you point me to what needs to be done to close this issue?

[colings86]

@s1monw Since frozen indices is not implemented this is no longer stalled so I'll remove the label. Thanks for pointing this out.

In terms of what needs to be done to have a freeze index action in ILM, we will need to implement a new ILM action and an AsyncActionStep to call the freeze API (it looks from the docs like the freeze API only returns when the index is frozen so I don't think we'll need a subsequent step to check the action has been completed like in other ILM actions). We will also need to update the allowed actions in the cold phase to allow the new freeze action. The ForcemergeAction might be a good template to use to see how an action is implemented.

Happy to answer any questions you have 😄

[talevy]

@simonw I'm happy to help.

[talevy]

implementation can be found in this PR: #36910