Add empty workflow for manually running canary in smithy-rs on behalf of external contributors (#3517)

## Motivation and Context
The same reason for #3500, which is GitHub actions do not allow us to
manually trigger them unless they are first checked-in to the main
branch.

## Description
This PR adds a skeletal workflow file for manually invoking canary
within `smithy-lang/smithy-rs` on behalf of external contributors.
Forked repositories will be missing repository secrets to run the canary
in CI so it will always fail for external contributors. As a workaround,
maintainers will manually trigger `manual-canary.yml`, given a PR number
from an external contributor as input.

A subsequent PR will fill in the blank.

----

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.github/workflows/manual-canary.yml

@@ -0,0 +1,18 @@
+# This workflow allows maintainers to manually run the canary given an external contributor's pull request created from
+# a forked repository, which does not have required repository secrets to execute the canary.
+name: Invoke Canary as Maintainer
+on:
+  workflow_dispatch:
+    inputs:
+      pull_request_number:
+        description: The PR number to invoke the canary for.
+        required: true
+        type: string
+
+jobs:
+  canary:
+    name: Canary
+    runs-on: ubuntu-latest
+    steps:
+    - name: Invoke canary
+      run: echo "Hello World"