Added 'type' to schema's Limit Object

[ignpelloz]

Often, proxies can be configured in two ways for handling requests and applying rate limiting: average and burst. For example, if an endpoint allows 10 requests per minute:

• Average: Would allow an average of 10 requests per minute, meaning only one request every 6 seconds would be accepted. Any other request received within these 6-second intervals would be rejected with HTTP code 429.
• Burst: Would allow all 10 requests to be served at the any time during the minute. Once 10 requests have been served and until the end of the minute, all successive requests would be rejected with HTTP code 429.

With this PR I have modified the schema to allow specifying on the SLA how the API handles requests targeting a given endpoint: Limit Object's type can be set to burst or average.

(cc @pafmon)