Chore: Dependency pinning & Depdendabot introduction (#436)

1. Pins the Github Action workflow hashes under `.github/workflows` to
full-length commit SHAs.
2. Adds a base level `.github/dependabot.yml` config to keep these (and
other go/Docker) deps up to date.

## Motivation and Context
- Github's [security
guide](https://docs.github.com/en/actions/reference/security/secure-use#using-third-party-actions)
recommends using the immutable SHA of an action when consuming third
party actions.
- Depdendabot makes ongoing dep house keeping manageable.

.github/workflows/deploy.yml

@@ -18,13 +18,13 @@ jobs:
       packages: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 #v3
 
       - name: Log in to Container Registry
-        uses: docker/login-action@v3
+        uses: docker/login-action@184bdaa0721073962dff0199f1fb9940f07167d1 #v3
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
@@ -32,7 +32,7 @@ jobs:
 
       - name: Extract metadata
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@c1e51972afc2121e065aed6d45c65596fe445f3f #v5
         with:
           images: ghcr.io/${{ github.repository }}
           tags: |
@@ -40,7 +40,7 @@ jobs:
             type=raw,value=main,enable={{is_default_branch}}
 
       - name: Build and push Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@ca052bb54ab0790a636c9b5f226502c73d547a25 #v5
         with:
           context: .
           file: ./Dockerfile
@@ -61,25 +61,25 @@ jobs:
       cancel-in-progress: false
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
       - name: Setup Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 #v5
         with:
           go-version: ${{ env.GO_VERSION }}
 
       - name: Setup Pulumi
-        uses: pulumi/actions@v6
+        uses: pulumi/actions@cc7494be991dba0978f7ffafaf995b0449a0998e #v6
         with:
           pulumi-version: ${{ env.PULUMI_VERSION }}
 
       - name: Authenticate to Google Cloud
-        uses: google-github-actions/auth@v2
+        uses: google-github-actions/auth@c200f3691d83b41bf9bbd8638997a462592937ed #v2
         with:
           credentials_json: ${{ secrets.GCP_STAGING_SERVICE_ACCOUNT_KEY }}
 
       - name: Setup Google Cloud SDK
-        uses: google-github-actions/setup-gcloud@v2
+        uses: google-github-actions/setup-gcloud@e427ad8a34f8676edf47cf7d7925499adf3eb74f #v2
         with:
           project_id: mcp-registry-staging
           install_components: gke-gcloud-auth-plugin
@@ -100,25 +100,25 @@ jobs:
       cancel-in-progress: false
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
       - name: Setup Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 #v5
         with:
           go-version: ${{ env.GO_VERSION }}
 
       - name: Setup Pulumi
-        uses: pulumi/actions@v6
+        uses: pulumi/actions@cc7494be991dba0978f7ffafaf995b0449a0998e #v6
         with:
           pulumi-version: ${{ env.PULUMI_VERSION }}
 
       - name: Authenticate to Google Cloud
-        uses: google-github-actions/auth@v2
+        uses: google-github-actions/auth@c200f3691d83b41bf9bbd8638997a462592937ed #v2
         with:
           credentials_json: ${{ secrets.GCP_PROD_SERVICE_ACCOUNT_KEY }}
 
       - name: Setup Google Cloud SDK
-        uses: google-github-actions/setup-gcloud@v2
+        uses: google-github-actions/setup-gcloud@e427ad8a34f8676edf47cf7d7925499adf3eb74f #v2
         with:
           project_id: mcp-registry-prod
           install_components: gke-gcloud-auth-plugin