core: support configurable openapi URLs

[bajtos]

Allow application developers to customize the URL where application's openapi spec is served (current hard-coded default: /openapi.json).

See #475 (comment)

[raymondfeng]

To serve OpenAPI spec with LB.next will be a good a validation for the following perspectives:

1. Allow a component (openapi explorer component) to serve certain routes
2. Support content negations based on Accept header and other information so that we can render OpenAPI spec in v2, v3, json, yaml etc.
3. Enable CORS

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[bajtos]

Removing from 4.0 GA to "Further rest layer improvements" epic (#1452).

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