`Microsoft.Web/sites` - `metadata` stack should be declared in swagger spec

Referencing https://github.com/Azure/bicep-types-az/issues/1968, `metadata` stack needs to be added to the swagger. Setting stack type via metadata updates stack type appropriately, but gives warnings because the property is not declared in the swagger.

cc @alex-frankel

Review the Microsoft.Web/sites swagger definition and the linked bicep-types-az issue first. Confirm the metadata stack declaration matches the requested shape and verify that setting stack type through metadata no longer produces undeclared-property warnings.

Written by the indexing model from the issue text.