Add empty workflow for manually running canary in smithy-rs on behalf of...

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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