neonvm: add pgbouncer patch to support DEALLOCATD/DISCARD ALL (#5958)

pgbouncer 1.21.0 doesn't play nicely with DEALLOCATD/DISCARD ALL if
prepared statement support is enabled (max_prepared_statements > 0).
There's a patch[0] that improves this (it will be included in the next 
release of pgbouncer).

This PR applies this patch on top of 1.21.0 release tarball. 
For some reason, the tarball doesn't include `test/test_prepared.py` 
(which is modified by the patch as well), so the patch can't be applied 
clearly. I use `filterdiff` (from `patchutils` package) to apply 
the required changes.

[0] a7b3c0a5f4

vm-image-spec.yaml

@@ -88,11 +88,12 @@ build: |
   RUN set -e \
       && apt-get update \
       && apt-get install -y \
-          curl \
           build-essential \
-          pkg-config \
+          curl \
           libevent-dev \
-          libssl-dev
+          libssl-dev \
+          patchutils \
+          pkg-config
 
   ENV PGBOUNCER_VERSION 1.21.0
   ENV PGBOUNCER_GITPATH 1_21_0
@@ -100,6 +101,7 @@ build: |
       && curl -sfSL https://github.com/pgbouncer/pgbouncer/releases/download/pgbouncer_${PGBOUNCER_GITPATH}/pgbouncer-${PGBOUNCER_VERSION}.tar.gz -o pgbouncer-${PGBOUNCER_VERSION}.tar.gz \
       && tar xzvf pgbouncer-${PGBOUNCER_VERSION}.tar.gz \
       && cd pgbouncer-${PGBOUNCER_VERSION} \
+      && curl https://github.com/pgbouncer/pgbouncer/commit/a7b3c0a5f4caa9dbe92743d04cf1e28c4c05806c.patch | filterdiff --include a/src/server.c | patch -p1 \
       && LDFLAGS=-static ./configure --prefix=/usr/local/pgbouncer --without-openssl \
       && make -j $(nproc) \
       && make install