[Writable Warm] File Pinning support in FileCache

[rayshrey]

Is your feature request related to a problem? Please describe

With FileCache now being used by Composite Directory in addition to Remote Snapshots we need to add additional functionalities.

One of the prominent functionalities needed is to add support for pinning a file in FileCache so that it becomes illegible for eviction and is not evicted unless unpinned.

Describe the solution you'd like

Once a file is pinned, it shouldn't be evicted from FileCache.
Will update this section with more details once have a concrete LLD for this.

Related component

Storage:Remote

Describe alternatives you've considered

No response

Additional context

No response

[peternied]

[Triage - attendees 1 2 3 4 5 6 7 8]
@rayshrey Thanks for creating this issue.