feat!: add schema union support

[jonaslagoni]

Description
This PR adds support for schema union support.

Related issue(s)
Related to asyncapi/spec#622
Fixes #809

[magicmatatjahu]

@jonaslagoni You shouldn't remove the schemaFormat() method from message object, why? In v2 people still can use that, so they should have possibility to retrieve that value. In v3 we should have still that method, but retrieving logic should be based on the message.payload.schemaFormat value :)

EDIT: Ok, you removed from message trait - but if there's a possibility to have that value in v2 message's trait, the v2 should return that, in v3 we should return undefined.

[fmvilas]

Looks good to me in general. Left a minor comment.

[fmvilas]

Suggested change

	const payloadIsSchemaUnion = this._json.payload.schemaFormat !== undefined;
	return payloadIsSchemaUnion ? this._json.payload.schemaFormat : getDefaultSchemaFormat(this._meta.asyncapi.semver.version);
	const payloadIsSchemaUnion = this._json.payload?.schemaFormat !== undefined;
	return payloadIsSchemaUnion ? this._json.payload.schemaFormat : getDefaultSchemaFormat(this._meta.asyncapi.semver.version);

I think payload can be undefined, right? We should be careful then.

[smoya]

About naming: asyncapi/spec-json-schemas#370 (comment)

[smoya]

Looks pretty good to me! Just few changes that are needed. Including the one @fmvilas requested in https://github.com/asyncapi/parser-js/pull/785/files#r1242462994

[smoya]

I'm taking over this one as @jonaslagoni is 🌴 🏖️

[smoya]

Required Parser-API changes are now in the following PR asyncapi/parser-api#100

cc @jonaslagoni @fmvilas

[sonarcloud]

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[smoya]

Since this PR has been opened for a while and lot of changes come from next-major-spec, and because I took over the work, I'm gonna close this one and instead open a new PR whenever I have a first working version.

cc @jonaslagoni