Access control per baggage item key

[ktoso]

This mirrors the slashmo/gsoc-swift-baggage-context#35 ticket

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Additional capability similar to privacy in os_log to be added to baggage keys.

A key has to define if it wants to be included in forEach and if it wants to be logged.

[...] mark a key as containing sensitive information . for example, you may have a key that carries sensitive user information and that you never want the collectors to log

via @tomerd

[ktoso]

Existing PR on old repo ktoso/gsoc-swift-baggage-context#2

[weissi]

This could be handled with static properties on the keys, right? We can default them to true but if you don't want them to be logged/enumerated/... you set them to false. Something like static var allowsEnumeration: Bool { return false }