* [ILM] Add date setting to calculate index age
Add the `index.lifecycle.origination_date` to allow users to configure a
custom date that'll be used to calculate the index age for the phase
transmissions (as opposed to the default index creation date).
This could be useful for users to create an index with an "older"
origination date when indexing old data.
Relates to #42449.
* [ILM] Don't override creation date on policy init
The initial approach we took was to override the lifecycle creation date
if the `index.lifecycle.origination_date` setting was set. This had the
disadvantage of the user not being able to update the `origination_date`
anymore once set.
This commit changes the way we makes use of the
`index.lifecycle.origination_date` setting by checking its value when
we calculate the index age (ie. at "read time") and, in case it's not
set, default to the index creation date.
* Make origination date setting index scope dynamic
* Document orignation date setting in ilm settings
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