Server side rejections based on Explicit Request Priority

[tushar-kharbanda72]

Is your feature request related to a problem? Please describe.
#1042 aims to build back-pressure support for Search requests. This feature will allow in selecting which requests to reject first if we have to reject any. This priority would be driven by the users

Describe the solution you'd like

Extending on #1184, with #1140 we propose customers also to have the ability to provide the priority of a requests, as a request field. The priority in the request is specified for a dynamic queue such as one of (CRITICAL, HIGHEST, HIGH, NORMAL, LOW, LOWEST). Back-pressure framework should be able to factor in this priority, and honour it while taking rejection decisions. This will provide deeper isolation within queries, while letting the low priority queries being rejected at first under duress, until there are serious reasons to reject high priority ones.

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