[Storage] Make the CORS rules nullable

[ShivangiReja]

In our (Blob/Queue/File)ServiceClient.SetPropertiesAsync method, allow the list of Cors rules to be null to indicate they should be ignored instead of cleared.

In order to do that, created a new swagger attribute "x-az-nullable-array" that we specify on these models and then allow null to prevent them from showing up in the generated code

[tg-msft]

Can we add a note about this new property at the top of https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/storage/Azure.Storage.Common/swagger/Generator/readme.md

I'm trying to keep track of all the extensions we've added.

[tg-msft]

Does this need to be isNullable? or can it just be isNullable?

[ShivangiReja]

I really wanted to not have this as an optional parameter but since TypeScript doesn't assign default values for primitive types, I had to make it as optional (microsoft/TypeScript#5113)

If I'd make isNullable required then we would have to set isNullable when we add headers and body in createResponseType() method.

We can set false in those cases but do we need to set isNullable when creating a response type?

[tg-msft]

Awesome!