Designate an int enum as "Flags"

[Some languages have decorators](https://learn.microsoft.com/en-us/dotnet/api/system.flagsattribute?view=net-7.0) that can add functionality to enums that are meant to be a set of bit fields. Will [CADL enums](https://microsoft.github.io/cadl/docs/language-basics/enums/) support a decorator for enums so that language generators can take advantage of this?

Start with the linked CADL enums documentation and the referenced FlagsAttribute behavior. Determine how enum decorators are represented and how language generators consume them. Done means the issue's intended decorator support and generator-facing semantics are specified and consistently supported.

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