Added statfs for block-cache

[ashruti-msft]

Added statfs for block_cache which will show the memory available if disk is not configured otherwise the disk space. Added corresponding tests for this and fixed a few unit changes in file_cache.

Used a VM with an attached disk of size 5G for the follwoing outputs:

When file cache/block cache uses the attached disk for disk caching

When config has block_cache component without disk cache it will show the memory available:

Output when file cache/block cache uses the same VM as disk cache

[jainakanksha-msft]

@ashruti-msft Please add the output in the PR description and details of the change.

[jainakanksha-msft]

@vibhansa-msft
This PR changes will represent temp disk details with the notation of root Filesystem.
I think that can confuse the users.
Should we change the FileSystem representation also for this somehow if that is feasible?

[ashruti-msft]

@ashruti-msft Please add the output in the PR description and details of the change.

@jainakanksha-msft I have added the output please have a look