Allow setting no-cors in ping service

[kmjayadeep]

Description

Currently Ping custom service does not work if the service is hosted under a different domain and no Access-Control-Allow-Origin header is set on the api response. This PR fixes this issue by allowing to make requests with mode: no-cors in fetch api.

More info about the mode field can be found here

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file

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[DanielBaulig]

@bastienwirtz Any chance we can get this merged? This would give us a great alternative to setting up proxy servers for fixing CORS related ping issues.