[swift6] Change Response to struct

[x-sheep]

This PR changes the Response open class to a struct. This is a breaking change, but I think there's a lot of benefits to not allowing Response to be overridden. The primary benefit is conformance to Sendable, which is important for Swift 6 specifically.

@jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

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[4brunu]

Looks good to me 👍
In case someone needs to override the response, we can introduce a flag to allow it.