[v3] Implement custom schema parsing

[smoya]

Describe the bug

I found out the following commented code https://github.com/asyncapi/parser-js/blob/next-major-spec/src/custom-operations/index.ts#L44-L47.

That means the call to parse() on custom schema parsers (Avro, OpenAPI, etc) is not being called for v3. That means schemas with those schema formats are never parsed. It also means users and tools won't be able to access to the transformed schema (we transform all of them to AsyncAPI Schema).

Expected behavior

The call to parse should be implemented.

cc @magicmatatjahu

[magicmatatjahu]

It wasn't implemented because we had to wait to final version of schemas (exactly multiple schemas) in v3

[smoya]

This is then completely critical and requires to be implemented for the next release, otherwise schemas with a schema format different than AsyncAPI one won't be parsed, meaning raw schemas will be in place.

[jonaslagoni]

/progress 90 finished and ready to be reviewed

[jonaslagoni]

Done in #864