[mgmt][typespec migration] `as_dict` behavior change for hierarchy model

[msyyc]

When try to migrate azure-mgmt-avs from swagger to typespec, I find the following breakings for hierarchy model:

• Before

from azure.mgmt.avs.models import CloudLink

model = CloudLink(linked_cloud="hello")
model_dict = model.as_dict(keep_readonly=False)
print(model_dict) # output: {'linked_cloud': 'hello'}

• After

from azure.mgmt.avs.models import CloudLink

model = CloudLink(linked_cloud="hello")
model_dict = model.as_dict(exclude_readonly=True)
print(model_dict) # output: {'properties': {'linkedCloud': 'hello'}}

We can see there are 3 breakings:

1. keep_readonly is renamed to exclude_readonly with reverse meaning
2. key is changed from snake_case to camelCase
3. old as_dict hide the hierarchy structure to follow model structure while new as_dict respect the hierarchy structure to keep consistent with REST API layer.

[msyyc]

1 and 2 are easy to fix for SDK users. While for 3, the breaking is relatively bigger since the output structure changes.

[msyyc]

proposal to mitigate the breakings to least : https://gist.github.com/iscai-msft/aeff44a16b1367f2807a7b8186f2cd00

[lmazuel]

This will be adressed as azure-core features

#41436
#41437
#41438
#41439
microsoft/typespec#7559