CI: Setup PR workflow

.github/workflows/build-linux.yml

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+name: (Windows) Build Plugin
+
+on:
+  workflow_call:
+    secrets:
+      DOCKER_TOKEN:
+        required: true
+      DOCKER_USERNAME:
+        required: true
+
+jobs:
+  linux-build-plugin:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        unreal: ['4.27', '5.0.2', '5.1']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Free disk space
+        if: ${{ steps.exists.outputs.value == 0 }}
+        run: |
+          # time df -h
+          sudo time swapoff -a
+          sudo time rm -f /swapfile
+          sudo time rm -rf /usr/local/lib/android
+          sudo time rm -rf /usr/share/dotnet
+          sudo time rm -rf /usr/share/swift
+          sudo time rm -rf /usr/local/.ghcup
+          sudo time rm -rf /usr/local/lib/node_modules
+          sudo time rm -rf /usr/lib/google-cloud-sdk
+          sudo time rm -rf /opt/pipx
+          sudo time rm -rf "$AGENT_TOOLSDIRECTORY"
+          sudo time apt-get clean
+          sudo time rm -rf /var/lib/apt/lists/*
+          # time docker rmi $(docker image ls -aq)
+          # time du --max-depth=3 --threshold=100M -h /usr /opt /var 2>/dev/null | sort -hr
+          df -h
+
+      - name: Run Docker container
+        run: |
+          echo ${{ secrets.DOCKER_TOKEN }} | docker login ghcr.io -u ${{ secrets.DOCKER_USERNAME }} --password-stdin
+          docker run -td --name unreal --user root \
+            -v ${{ github.workspace }}:/ue4-targetsystemplugin \
+            --workdir /ue4-targetsystemplugin/TargetSystem \
+            ghcr.io/epicgames/unreal-engine:dev-slim-${{ matrix.unreal }}
+
+      - name: Installing Linux Dependencies
+        run: |
+          docker exec unreal sudo apt-get update
+          docker exec unreal pip3 install --upgrade pip
+
+      - name: Installing UE CLI
+        run:
+          docker exec unreal pip3 install ue4cli
+
+      - name: Configuring UE CLI
+        run:
+          docker exec unreal ue4 setroot /home/ue4/UnrealEngine
+
+      - name: Package plugin
+        run: docker exec unreal ue4 package

.github/workflows/pr-windows.yml

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+name: (Windows) Build Plugin
+
+on:
+  workflow_call:
+    secrets:
+      WIN_UNREAL_ENGINE_ROOT:
+        required: true
+
+jobs:
+  windows-build-plugin:
+    name: Build & Test
+
+    if: github.actor == 'mklabs'
+    runs-on: [self-hosted, Windows]
+
+    strategy:
+      matrix:
+        unreal: ['4.27', '5.0.2', '5.1']
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v3
+        with:
+          node-version: 18
+
+      - name: Store prev ue4 cli root
+        id: vars-ue4
+        run: >-
+          echo "::set-output name=prev_ue4_cli_root::$(ue4 root)"
+
+      - name: Configuring UE CLI
+        run:
+          ue4 setroot "${{ secrets.WIN_UNREAL_ENGINE_ROOT }}UE_${{ matrix.unreal }}"
+
+      - name: Package plugin
+        run: ue4 package
+        working-directory: TargetSystem
+
+      # TODO: Sanity checks automated testing (missing unit / functional tests to run as of now)
+
+      - name: Restore UE CLI Configuration
+        if: always()
+        run:
+          ue4 setroot "${{ steps.vars-ue4.outputs.prev_ue4_cli_root }}"

.github/workflows/pr.yml

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+name: PR - Build Plugin & Run Tests
+
+on:
+  pull_request:
+    branches: [ master, dev, dev-ue5 ]
+
+concurrency: 
+  group: pr-${{ github.head_ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-on-pr-windows:
+    name: (Windows) Build
+    uses: ./.github/workflows/pr-tests-windows.yml
+    secrets:
+      WIN_UNREAL_ENGINE_ROOT: ${{ secrets.WIN_UNREAL_ENGINE_ROOT }}
+
+  build-linux:
+    name: (Linux) Build
+    uses: ./.github/workflows/build-linux.yml
+    secrets:
+      DOCKER_TOKEN: ${{ secrets.DOCKER_TOKEN }}
+      DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}