`Microsoft.CognitiveServices/accounts` - `kind` values should be modeled as an enum

The `kind` property defined for an "accounts" resource is modeled as a string with no information on valid values. It would be useful to model as an enum so that the accepted values can be enumerated:

https://github.com/Azure/azure-rest-api-specs/blob/88e7838a09868a51de3894114355c75929847a46/specification/cognitiveservices/resource-manager/Microsoft.CognitiveServices/stable/2021-04-30/cognitiveservices.json#L1091-L1094

Initially reported under https://github.com/Azure/bicep/issues/4942

Start with specification/cognitiveservices/resource-manager/Microsoft.CognitiveServices/stable/2021-04-30/cognitiveservices.json at lines 1091-1094, then inspect nearby resource definitions for enum modeling conventions. Confirm the accepted kind values from the specification context and update the model so those values are enumerated; done means the accounts resource no longer exposes kind as an unconstrained string.

Written by the indexing model from the issue text.