ci: auto-create GitHub Release on version bump to main

Adds a workflow that detects a change to __version__ in
awslambdaric/__init__.py on main, then creates a tag and GitHub Release
using notes from RELEASE.CHANGELOG.md. Skips if the release already exists.

Author: M-Elsaeed

.github/workflows/release-on-version-change.yml

@@ -0,0 +1,74 @@
+name: release-on-version-change
+
+# Creates a GitHub Release automatically whenever the package version in
+# awslambdaric/__init__.py changes on main. The release notes are taken from
+# the matching section of RELEASE.CHANGELOG.md.
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - 'awslambdaric/__init__.py'
+
+permissions:
+  contents: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Read version
+        id: version
+        run: |
+          VERSION="$(grep -Po '__version__\s*=\s*"\K[^"]+' awslambdaric/__init__.py)"
+          if [ -z "$VERSION" ]; then
+            echo "Could not determine version from awslambdaric/__init__.py" >&2
+            exit 1
+          fi
+          echo "version=$VERSION" >> "$GITHUB_OUTPUT"
+          echo "Detected version: $VERSION"
+
+      - name: Check if release already exists
+        id: check
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          if gh release view "${{ steps.version.outputs.version }}" >/dev/null 2>&1; then
+            echo "exists=true" >> "$GITHUB_OUTPUT"
+            echo "Release ${{ steps.version.outputs.version }} already exists; skipping."
+          else
+            echo "exists=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Extract changelog notes
+        if: steps.check.outputs.exists == 'false'
+        id: notes
+        run: |
+          VERSION="${{ steps.version.outputs.version }}"
+          NOTES="$(awk -v ver="$VERSION" '
+            index($0, "`" ver "`") == 1 { capture = 1; next }
+            capture && /^### / { exit }
+            capture { print }
+          ' RELEASE.CHANGELOG.md)"
+          # Trim leading/trailing blank lines
+          NOTES="$(printf '%s\n' "$NOTES" | sed -e '/./,$!d' | tac | sed -e '/./,$!d' | tac)"
+          if [ -z "$NOTES" ]; then
+            NOTES="Release $VERSION"
+          fi
+          {
+            echo "body<<EOF"
+            echo "$NOTES"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Create GitHub Release
+        if: steps.check.outputs.exists == 'false'
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          VERSION="${{ steps.version.outputs.version }}"
+          gh release create "$VERSION" \
+            --target "${{ github.sha }}" \
+            --title "AWS Lambda Runtime Interface Client for Python v$VERSION" \
+            --notes "${{ steps.notes.outputs.body }}"