fix: ensure IoT Hub list persists over DPS enrollment (group) updates

[vilit1]

With respect to Azure/azure-cli#29002

Fixing some weirdness in DPS enrollments and enrollment groups:

1. fix the iot hub param to allow multiple uses

create a new enrollment group with 2 iot hubs (here I just use the same one):

2. ensure that update will keep the iot hub list unless a new list is given

re-use the enrollment; update the allocation policy and see that the hubs are still there:

try to update to static, get expected failure because there are 2 hubs:

get it to work when a single hub is defined:

3. minimize the number of inputs needed to update custom allocation policy enrollments

going from static to custom, all inputs are still needed:

only want to update the url:

tests

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