Be able to get number of CPUs (#3774)

After enabling autoscaling, we faced the issue that customers are not
able to get the number of CPUs they use at this moment. Therefore I've
added these two options:

1. Postgresql function to allow customers to call it whenever they want
2. `compute_ctl` endpoint to show these number in console

Dockerfile.compute-node

@@ -409,6 +409,10 @@ COPY pgxn/ pgxn/
 RUN make -j $(getconf _NPROCESSORS_ONLN) \
         PG_CONFIG=/usr/local/pgsql/bin/pg_config \
         -C pgxn/neon \
+        -s install && \
+    make -j $(getconf _NPROCESSORS_ONLN) \
+        PG_CONFIG=/usr/local/pgsql/bin/pg_config \
+        -C pgxn/neon_utils \
         -s install
 
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