Need a new url to retrieve all authorized sku Names of MySQL/MariaDB/PostgreSQL servers

We have the feedback from Azure Powershell users that
"The values to use for the SKU parameter were not intuitive and if a non-supported value is used, the error should be explicit and if possible, list the authorized values."

Since Powershell side has no such information, we request the service teams to provide a new url to retrieve such information for MySQL/MariaDB/PostgreSQL servers.

Please refer to the following swagger for the similar operation in compute module.
https://github.com/Azure/azure-rest-api-specs/blob/c81039d056cc5aa0a0025f5b9e5446a7c194bd1f/specification/compute/resource-manager/Microsoft.Compute/stable/2019-12-01/compute.json#L4180

```
"/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Compute/virtualMachineScaleSets/{vmScaleSetName}/skus": {
"get": {
"tags": [
"VirtualMachineScaleSets"
],
"operationId": "VirtualMachineScaleSets_ListSkus",
"description": "Gets a list of SKUs available for your VM scale set, including the minimum and maximum VM instances allowed for each SKU.",
"parameters": [
{
"name": "resourceGroupName",
"in": "path",
"required": true,
"type": "string",
"description": "The name of the resource group."
},
{
"name": "vmScaleSetName",
"in": "path",
"required": true,
"type": "string",
"description": "The name of the VM scale set."
},
{
"$ref": "#/parameters/ApiVersionParameter"
},
{
"$ref": "#/parameters/SubscriptionIdParameter"
}
],
"responses": {
"200": {
"description": "OK",
"schema": {
"$ref": "#/definitions/VirtualMachineScaleSetListSkusResult"
}
}
},
"x-ms-pageable": {
"nextLinkName": "nextLink"
}
}
},
```

Start with the referenced compute swagger operation, VirtualMachineScaleSets_ListSkus, and compare its path, parameters, response, and paging behavior with the MySQL, MariaDB, and PostgreSQL service specifications. Done means the relevant service specifications define a documented URL for retrieving authorized SKU names across all three server types.

Written by the indexing model from the issue text.