accessor methods for getting information about the OpenAPI description

[karenetheridge]

Recall that the "OpenAPI description" is the sum total of all the openapi documents known to the implementation, and the APIs described by them.

• openapi_version -> $schema->{openapi}
• api_title, api_version, ... -> $schema->{info}{...}
• operationIds -> the list of all known operationIds
• paths -> the list of all path templates
• apis -> tuples consisting of [ method, path_template ]
• security_schemes ...
• servers ...