Add empty workflow for running canary in `aws-sdk-rust` from `smithy-rs` CI (#3500)

## Motivation and Context
Add a skeletal workflow file for later invoking canary in `aws-sdk-rust`
from within `smithy-rs` CI. GitHub actions do not allow us to manually
trigger them unless they are first checked-in to the main branch.

(**Disclaimer**: we may end up executing `canary-runner` from a
smithy-rs CI step without invoking canary in `aws-sdk-rust`, but for the
sake of illustration, assume we invoke canary in `aws-sdk-rust` for the
rest of the PR description)

## Description
To briefly explain where we're going, a diagram below illustrates how
relevant workflow files look like when we run canary in `smithy-rs` CI
(in the context of this PR, it will check-in a file that says
`canary.yml` in red).

<p align="center">
<img width="600" alt="Screen Shot 2023-02-08 at 3 47 05 PM"
src="https://github.com/smithy-lang/smithy-rs/assets/15333866/4a336f7b-8d41-42e4-b59d-4aa87ab91093">
</p>

We will add a new CI step that runs canary in `aws-sdk-rust` from within
`smithy-rs`. It will work when run as part of `cr-pr.yml` but it won't
when run from `cr-pr-forks.yml` (i.e. for external contributors) because
their forked repos do not have a necessary repository secret to invoke
canary in `aws-sdk-rust`. To work around, we will manually run
`smithy-lang/smithy-rs/.github/workflows/canary.yml` on their behalf
with the name of a fork and a commit hash.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._