[Enhancement] Define SC pricing name as enum

- Field schema: https://github.com/Azure/azure-rest-api-specs/blob/3695f4ef3bf65c253509d2708c31c96b71822613/specification/security/resource-manager/Microsoft.Security/stable/2018-06-01/pricings.json#L223

- What happened?
The `pricingName` path argument in the Security Center pricing endpoints version 2018-06-01 (https://docs.microsoft.com/en-us/rest/api/securitycenter/pricings/get) accepts the following values: `AppServices`, `ContainerRegistry`, `KeyVaults`, `KubernetesService`, `SqlServers`, `SqlServerVirtualMachines`, `StorageAccounts`, `VirtualMachines`. However, this is not defined in the API specs.

- What did you expect or want to happen?
Ideally `pricingName` should be defined as an enum, similarly to `pricingTier`. https://github.com/Azure/azure-rest-api-specs/blob/3695f4ef3bf65c253509d2708c31c96b71822613/specification/security/resource-manager/Microsoft.Security/stable/2018-06-01/pricings.json#L197

Start by reading the pricingName path argument in specification/security/resource-manager/Microsoft.Security/stable/2018-06-01/pricings.json at the referenced line, then compare it with the pricingTier definition around line 197. Confirm the eight documented values and validate that pricingName is represented as an enum; done means the API specification reflects those allowed values.

Written by the indexing model from the issue text.