Add support for composed avro schemas

[ignacio-nyt]

Reason/Context

Please try answering few of those questions

• Why we need this improvement?
  It would be very useful for existing AVRO schemas that take advantage of this feature: https://callistaenterprise.se/blogg/teknik/2019/09/24/avro-schemas-and-compositionality/

• How will this change help?
  It would help bring existing pure AVRO ecosystems into the AsyncAPI world

• What is the motivation?
  Be able to take advantage of AsyncAPI

Description

Please try answering few of those questions

• What changes have to be introduced?
  AsyncAPI would need to be able to somehow load AVRO schemas and figure out the dependency tree

• Will this be a breaking change?
  Don't have enough info to answer this question

• How could it be implemented/designed?
  Don't have enough info to answer this question

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