[FEATURE] get-manifest hook should remove protocol from outgoingDomains if present

[filmaj]

Very annoying sharp edge: if you happen to include the protocol in outgoingDomain entries, you will get a cryptic error message when you go to deploy or run your app:

➜ slak deploy
The following error was returned by the apps.manifest.validate Slack API method

🚫 The provided manifest file does not validate against schema. Consult the additional errors field to locate specific issues (invalid_manifest)
   input must match regex pattern: ^(?![\.\-])([-a-zA-Z0-9\.])+([a-zA-Z0-9])$ (failed_constraint)
   Source: /outgoing_domains/0

In this case, I had outgoingDomains: ['https://google.com'] in my manifest. Dropping https:// fixes the issue.

My suggested solution is for the get-manifest hook to parse these entries using the URL module, and extract only the hostname, dropping protocol. Feels like it belongs squarely in the cleanManifest function of the get-manifest hook.

[filmaj]

This was filed on the deno-slack-sdk here: slackapi/deno-slack-sdk#245
And there's even an open PR for it here, but couldn't get the contributor to get that PR over the finish line: slackapi/deno-slack-sdk#246