[MPG] @format("uuid") should be generated as UUID in mgmt plane for brownfield

[XiaofeiCao]

diff in generated ManagedServiceIdentity(principalId and tenantId are both affected):

XiaofeiCao/azure-sdk-for-java@9bef365#r1602724257

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1. [Bug]: principalId and tenantId in ManagedServiceIdentity should be of type uuid ARM consistency typespec-azure#845
   bug lib:azure-resource-manager +2
   
2. In ARM, we should generate UUID for @format("uuid")

[XiaofeiCao]

TBD. We may break it to generate String even for UUID.