Pass API schema via URL

[Olegt0rr]

In some cases API schema is available on URL.
Please add a feature with passing an URL instead of mounting a file.

[d0znpp]

Add support of the OpenAPI spec loading via URL. From now on it's possible to set openapi specification URL to env var APIFW_API_SPECS.
Add custom Content-Type of the request to introspection endpoint. Custom content-type could be set to the env var APIFW_SERVER_OAUTH_INTROSPECTION_CONTENT_TYPE.

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#40

[d0znpp]

#40