[MPG] Backward compat access to "properties" flattening (basic scenario)

[lmazuel]

DPG models from TypeSpec won't flatten. To simplify backward compat for msrest to DPG models, we'll introduce:

If a model has a properties attribute, any direct access from the root to a property under properties should work.

For instance:

m.properties.foo # Returns "foo" in properties
m.foo # Also returns "foo" from properties

In the rare case where there is a name conflict, the local attribute should be returned.

Only for MPG (Mgmt from TypeSpec)

[lmazuel]

Let's do typeProperties as well

[msyyc]

new design: https://microsoft.sharepoint.com/:fl:/s/d40daadc-c0b7-46ef-8088-d7b3a5bad5f8/EYoU_oTaLERJrQkzXpeqe4gBdSZi4K-_jIsZ8_aI6c_ebQ?e=nxXelE&nav=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