Add 'passthrough' option to Image Resizer

Status: **Opened for review**

Reviewers: (pending)
JIRA: **https://mobify.atlassian.net/browse/PROD-1562**

## Changes
- Adds a `passthrough` option to Image Resizer that serves images through the IR backend without resizing them.

## How to test-drive this PR
- Run the tests and examples