Make Schema a superset of JSON Schema Draft 07

[fmvilas]

Fixes #212

@philsturgeon would you mind having a look at this PR? I took yours and changed/added some stuff.

Clarifications:

• XML Object and xml property have been removed. Future versions of AsyncAPI will support XSD, which is better suited for XML definitions.
• We're not moving to Draft-08 yet because of the lack of tooling and, IIRC, Draft-08 is still not released. However, we're doing steps in this direction by adding deprecated to our superset. Once Draft-08 is out and tooling implemented it, we can get rid of this property.
• We still support OpenAPI schemas on message payloads. Just indicate schemaFormat is application/vnd.oai.openapi;version=3.0.0. Actually, once OpenAPI gets rid of their current Schema object and evolves into a Draft-08 superset, we will be able to align again.

[WaleedAshraf]

the payload can still be XML serializable?

https://github.com/asyncapi/asyncapi/pull/256/files#diff-c579311543db4dde2bd2e2ca895ffed1R76

[fmvilas]

Oh yeah! The payload can be serialized to anything. We're just removing a feature (originally from OpenAPI schemas) that allows you to define XML using JSON Schema-like syntax. I'm doing this on purpose to keep it simple. If people request this feature in the future we can consider adding it. Adding is easy and quick but removing it later would be a mess and would require a major version bump.

It's on the roadmap for the next minor version to add support for non-JSON schema languages: XSD, Protobuf, etc. XSD is what most people using XML use for schema contracts.

[fmvilas]

Also, people can still use OpenAPI schemas if they need it. What's being changed here is the default schema format.

[fmvilas]

I'm moving forward and gonna merge this PR. Please, if you see something wrong, missing, or just don't agree with the changes, don't hesitate to leave a comment after it's merged. I'll be happy to address these questions afterward.

[fmvilas]

Thanks, @philsturgeon, and @WaleedAshraf for reviewing it.

[asyncapi-bot]

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