[FEATURE] Add dry-run option for ism actions

[aggarwalShivani]

Is your feature request related to a problem?
Currently, when creating an ism policy, user can't test directly if the configuration is working as expected.
They can only trigger the policy, let the action fail in the policy to see if the configs are not fine.

In elasticsearch-curator, there is a supported parameter for this:
dry-run: It can help you see which indices will be caught by operations without actually performing those operations.

What solution would you like?
Can we have a similar dry-run option added in ism as well? Perhaps adding it at the policy level, or for every action that just mentions the matched indices but not perform the action?
(There was a similar tkt raised for notification action alone ,but i ask this as a general feature)
Does this seem like a value-addition for ism? Pls let us know your thoughts!

Do you have any additional context?
Dry-run option in elasticsearch-curator

[dblock]

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