Add `join` command to redocly/openapi-core to allow joining from JS scripts

[richiksc]

Is your feature request related to a problem? Please describe. Hello - I have some pipelines in JS where I need to merge OpenAPI specs and then bundle with decorators, and right now this involves calling openapi join through child_process.exec.

Describe the solution you'd like It would be good if the join feature was added to openapi-core so this could be handled directly in JS.

Describe alternatives you've considered https://www.npmjs.com/package/openapi-merge