Reject unsupported content-types on webhooks

[stuartc]

Validation Steps

1. Run your local server
2. Get a web hook url for any workflow
3. Run a curl command:

curl \
 -X POST \
 -H "Accept: */*" \
 -H "Accept-Encoding: gzip, deflate" \
 -H "Content-Length: 4834" \
 -H "Content-Type: text/xml" \
 $WEBHOOK_URL

You should see: {"error":"Unsupported Media Type"} as the response.

Notes for the reviewer

Users that send data without a supported mime type cause exceptions to be raised because the controller tries to encode the request body (which is Unfetched).

Instead of throwing an exception, we should return a 415 error.

Related issue

Fixes #2180

Review checklist

• [x] I have performed a self-review of my code
• [x] I have verified that all appropriate authorization policies (:owner, :admin, :editor, :viewer) have been implemented and tested
• [ ] If needed, I have updated the changelog
• [ ] Product has QA'd this feature

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.70%. Comparing base (ec9e9cd) to head (1b01055).

:exclamation: Current head 1b01055 differs from pull request most recent head c390e3d

Please upload reports for the commit c390e3d to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2186 +/- ## ======================================= Coverage 89.69% 89.70% ======================================= Files 275 275 Lines 9036 9039 +3 ======================================= + Hits 8105 8108 +3 Misses 931 931 ```

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