Create backport PRs automatically (#3558)

## Motivation and Context
Automatically opens backport PRs merging
`smithy-rs-release-1.x.y-release` to `main`.

## Description
A workflow `backport-pull-request.yml` can be called in two ways:
- called automatically when a prod release runs
- called manually when a patch fix has been made to
`smithy-rs-release-1.x.y-release` and needs to be back-ported

## Testing
- Triggered the workflow manually and confirmed [this (dummy)
PR](https://github.com/smithy-lang/smithy-rs/pull/3556) got created
- Triggered the workflow automatically and confirmed [this (dummy)
PR](https://github.com/smithy-lang/smithy-rs/pull/3557) got created

----

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