Add availability monitor

Adds a product availability monitor.

Request Body — REQUIRED
SiteId int32
LanguageCode string
Email string
SkuId int32

Responses
200 Ok. Schema — OPTIONAL Message string — OPTIONAL A status message for the action taken. Details string[] — OPTIONAL Any validation messages for the data on the current action.
400 Bad request. Schema — OPTIONAL Message string — OPTIONAL A status message for the action taken. Details string[] — OPTIONAL Any validation messages for the data on the current action.