[swift6] Add Identifiable conformance to supported models

[x-sheep]

This PR adds a new compiler option (enabled by default) that adds conformance to the Identifiable protocol to models that have an id property of type string, UUID or any integer. Specs can override this per-model by adding the x-swift-identifiable extension.

Resolves #20143

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

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[4brunu]

Looks good to me 👍
Thanks for the PR.