chore(lambda-layer-proxyagent): autobump mechanism

Add an autobump mechanism similar to the one in
#18780.

Author: rix0rrr

Diff for: packages/@aws-cdk/lambda-layer-node-proxy-agent/build-tools/bump-proxyagent-version.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+set -eu
+scriptdir=$(cd $(dirname $0) && pwd)
+
+package=proxy-agent
+
+npm show $package version > $scriptdir/../proxyagent.version
+
+echo "Proxy Agent is currently at $(cat $scriptdir/../proxyagent.version)"

Diff for: packages/@aws-cdk/lambda-layer-node-proxy-agent/layer/Dockerfile

@@ -1,7 +1,7 @@
 # base lambda image
 FROM public.ecr.aws/lambda/nodejs:latest
 
-ARG PROXY_AGENT_VERSION=5.0.0
+ARG PROXY_AGENT_VERSION=0.0.0
 
 USER root
 RUN mkdir -p /opt

Diff for: packages/@aws-cdk/lambda-layer-node-proxy-agent/layer/build.sh

@@ -3,11 +3,13 @@ set -euo pipefail
 
 cd $(dirname $0)
 
-echo ">> Building AWS Lambda layer inside a docker image..."
+version=$(cat ../proxyagent.version)
+
+echo ">> Building AWS Lambda layer inside a docker image for Proxy Agent ${version}..."
 
 TAG='aws-lambda-node-proxy-agent'
 
-docker build -t ${TAG} .
+docker build -t ${TAG} . --build-arg PROXY_AGENT_VERSION=${version}
 
 echo ">> Extrating layer.zip from the build container..."
 CONTAINER=$(docker run -d ${TAG} false)

Diff for: packages/@aws-cdk/lambda-layer-node-proxy-agent/proxyagent.version

@@ -0,0 +1 @@
+5.0.0