Support access decorator at the model property level

We currently don't support TCGC's [access decorator](https://azure.github.io/typespec-azure/docs/libraries/typespec-client-generator-core/reference/decorators/#@Azure.ClientGenerator.Core.access) at the model property level. We should consider supporting it as it is a valid use case to want to do this via client.tsp vs writing custom code to achieve the same thing. Currently, TCGC only supports `public` or `internal`.