Add support for using $ref and local files at any place in the asyncapi file

[eivindga]

Maintaining one big asyncapi-file with markdown, examples etc. is a major headache when it starts to grow. We have a lot of apis and developers who make frequent changes, and this causes a lot of additional work.

Please consider adding support for $ref (or another tag) to include local external files at any place in the structure, to improve maintainability.

Also see issue:
asyncapi/generator#963

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[jonaslagoni]

Additional related issue: #778

[jonaslagoni]

Related to #868

[berry120]

Adding my use case to this one - we have one team who maintains a lot of separate APIs, and so there's many common elements between the different asyncapi docs that have to be duplicated (or pre-processed separately) without this functionality. Things like team contacts, reusable components like the server security scheme, etc.

[amelluk]

Related to asyncapi/parser-js#797 - adding my use case too. this feature is essential for us, when describing large apis

[baxterjo]

Adding my use case to this

[jonaslagoni]

Similar issue to #778

[smoya]

Closing in favour of #778