Validate YAML pipeline based on the JSON schema

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Schema: https://github.com/microsoft/azure-pipelines-vscode/blob/main/service-schema.json

We can validate this in different stages of serialization, publishing or just our tests

[premun]

Probably not as important issue though

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We could do this in our CI only maybe

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I don't see value in this as this is not causing any problems nor will bring any large benefit