Support for custom code samples via `x-codeSamples`

[rattrayalex]

Is your feature request related to a problem?

Yes, I would like docs to render with snippets that show use of an API's custom-built SDKs, rather than generic http requests. This is what Stripe does.

Describe the solution you'd like

If x-codeSamples is present, as with redocly, render the code present there rather than the code that would be generated by default.

Describe alternatives you've considered

n/a

Additional context

Previously discussed at #153

I might be willing to fund a PR for this if it would be accepted. Please share any requirements for it.

[rattrayalex]

@sserrata would you accept a PR adding this?

[sserrata]

Addressed in:

#697
#701
#706
#707