[Subscription RP] Add AdditionalParameters property to SubscriptionCreationParameters

[wilcobmsft]

This property will be used for Microsoft-internal subscription creation scenarios.

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[wilcobmsft]

CC @lmazuel as he's reviewed previous PRs for this swagger.

[lmazuel]

@olydis if you don't mind, I indeed reviewed the previous one and even suggested to use additionalProperty on an email thread, I steal you this review ;)

[lmazuel]

@annatisch FYI

[lmazuel]

You have an actual "additionalParameters" node in your JSON? Could you update an example with this node, I'm always more confident when I see an example if "additionalProperties" is used (regular source of trouble :))

[wilcobmsft]

Looks like I messed it up too. :) Added an example and made sure it's passing validation.