[Tiered caching] [Milestone 1] Framework changes

[sgup432]

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This issue is around framework/skeleton changes for tiered caching.

• Includes defining appropriate interfaces ie CachingTier etc
• TieredCachingService which will serve as a main class to handle tiered logic.
• Plugging tieredCachingService into IndicesRequestCache.

Refer main issue for more details - #10024
and #10870
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