Adding optional SLA definitions with the Spec

Related to #51

We have identified in the market a need for measuring and enforcement SLA in APIs. Specially, API vendors would benefit from an standard import of SLA metrics to measure and track importing them along with the OpenAPI definition in an standard way.

In this line, we have started a R&D project by Icinetic and the University of Seville at Spain to work on: "Extension on OpenAPI for the definition of Service Level Agreements (SLA)".

The project is looking forward to:

1. Setup a basic and optional profile extension to describe a set of plan prices over APIs, Quota enforcement policies, constraints, etc.
2. This information can (always optionally) decorate the OpenAPI spec inline on in a side file/resource linked from the main spec.
3. Third party tools like Hivepod: can generate both: Swagger spec, service implementation, and SLA policy + SLA enforcement code, data repository to store the metrics and tools for analysis
4. Third party tools like API Management tools can use this SLA info also to measure and track the level of compliant with respect this SLA specified.
5. The ISA Research Group in the University of Seville is looking forward to provide tools for validation, creation and verification of SLAs for free.
6. We will provide a reference implementation showing the full scenario: from sample Microservice being measured, using a data store to gather metrics and an analysis tool to visualize the SLA grade of compliance.
7. Eventually, we would love to contribute to OAI (next version) with an open source extension for SLA definition.

Looking forward for early feedback and to join forces with OAI wanting to support an extension for this in an standardized way.

Start by reviewing related issue #51 and the OpenAPI specification context described here. Define the optional profile, inline or linked representation, and validation or tooling scope before proposing an extension; done would be an agreed specification and reference implementation scope.