Add Motoko-variant of vetKD example dapp (#602)

Adds a variant of the rust/vetkd example dapp where the backend is implemented in Motoko, rather than Rust.

The code is almost entirely a 1:1 copy of the code in `rust/vetkd`, with the main difference that the `motoko/vetkd/src/app_backend` is no longer implemented in Rust but in Motoko.

.github/workflows/motoko-vetkd-example.yml

@@ -0,0 +1,43 @@
+name: motoko-vetkd
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    paths:
+      - motoko/vetkd/**
+      - .github/workflows/provision-darwin.sh
+      - .github/workflows/provision-linux.sh
+      - .github/workflows/motoko-vetkd-example.yml
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  motoko-vetkd-darwin:
+    runs-on: macos-12
+    steps:
+      - uses: actions/checkout@v1
+      - name: Provision Darwin
+        env:
+          DFX_VERSION: 0.14.2
+        run: bash .github/workflows/provision-darwin.sh
+      - name: Motoko vetKD Darwin
+        run: |
+          dfx start --background
+          pushd motoko/vetkd
+          make test
+          popd
+  motoko-vetkd-linux:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v1
+      - name: Provision Linux
+        env:
+          DFX_VERSION: 0.14.2
+        run: bash .github/workflows/provision-linux.sh
+      - name: Motoko vetKD Linux
+        run: |
+          dfx start --background
+          pushd motoko/vetkd
+          make test
+          popd

.github/workflows/motoko-vetkd-skip.yml

@@ -0,0 +1,16 @@
+name: motoko-vetkd
+on:
+  pull_request:
+    paths-ignore:
+      - motoko/vetkd/**
+      - .github/workflows/provision-darwin.sh
+      - .github/workflows/provision-linux.sh
+      - .github/workflows/motoko-vetkd-example.yml
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo 'Not needed - relevant folder not touched'

motoko/vetkd/.gitignore

@@ -0,0 +1,24 @@
+# Various IDEs and Editors
+.vscode/
+.idea/
+**/*~
+
+# Mac OSX temporary files
+.DS_Store
+**/.DS_Store
+
+# dfx temporary files
+.dfx/
+
+# rust
+target/
+
+# generated files
+src/declarations/
+
+# frontend code
+node_modules/
+dist/
+
+# environment variables
+.env