Add counter for total file handles in KQP spilling

[spuchin]

We need to reduce our usage of file handles in spilling, as at the moment we can reach system limit, which cannot be configured inside YDB:
#11637

As a first step, we need to actually monitor our file handles usage in spilling, so I propose we add sensor for total usage per node, similar to other spilling counters:

ydb/ydb/library/yql/dq/actors/spilling/spilling_counters.h

Line 13 in 27eb560

::NMonitoring::TDynamicCounters::TCounterPtr SpillingWriteBlobs;