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/ci.yml

@@ -16,15 +16,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v4
+      uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
     - name: Set up Go
-      uses: actions/setup-go@v5
+      uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 #v5
       with:
         go-version: ${{ env.GO_VERSION }}
 
     - name: Cache Go modules
-      uses: actions/cache@v4
+      uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809 #v4
       with:
         path: |
           ~/.cache/go-build
@@ -60,15 +60,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout code
-      uses: actions/checkout@v4
+      uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
     - name: Set up Go
-      uses: actions/setup-go@v5
+      uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 #v5
       with:
         go-version: ${{ env.GO_VERSION }}
 
     - name: Cache Go modules
-      uses: actions/cache@v4
+      uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809 #v4
       with:
         path: |
           ~/.cache/go-build
@@ -84,7 +84,7 @@ jobs:
       run: make test-all
 
     - name: Upload coverage artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 #v4
       with:
         name: coverage-report
         path: |