[RFC] Pulse (#57108)

* [RFC][skip-ci] Pulse

* Add drawback

* Add Opt-In/Out endpoint

* Add clarification about synched local internal indices

* Update rfcs/text/0008_pulse.md

Co-Authored-By: Josh Dover <[email protected]>

* Add Phased implementation intentions, Security and Integrity challenges and example of use

* Refer to a follow up RFC to talk about security in the future

* Fix wording + add Legacy behaviour

Co-authored-by: Elastic Machine <[email protected]>
Co-authored-by: Josh Dover <[email protected]>