Add availability blocks

Adds availability blocks which are used to group related Availability updates. This makes limiting public availability easier and more organized. Each block specifies a TemplateRateId - the Rate used as a template for the block. When the block is created, a new dedicated Rate is created based on this template, so the RateId returned in the response will differ from the TemplateRateId provided in the request. Reservations picked up from the block inherit pricing from the newly created rate. Note this operation supports Portfolio Access Tokens.