[Extensions] Introduce ClientBehaviour model

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Extension processing also **alter platform behaviour**. As an example [12.1. Credential Protection (credProtect)](https://github.com/linux-credentials/libwebauthn/pull/77#issuecomment-2674105649) may require performing additional authenticator checks, in certain circumstances. The need for these checks may be represented in a new `ClientBehaviour` struct, to be potentially altered during extension processing:
```rust
trait ClientExtensionProcessing {
fun get(&self, input: &G, &mut clientBehaviour: ClientBehaviour): ExtensionResult
fun create(&self, input: &C, &mut clientBehaviour: ClientBehaviour): ExtensionResult
}
```

_Originally posted by @AlfioEmanueleFresta in https://github.com/linux-credentials/libwebauthn/issues/77#issuecomment-2674105649_

Start by locating the ClientExtensionProcessing trait and the current extension-processing implementations; compare how get and create receive their inputs. Define the scope and representation of ClientBehaviour, then verify that extension processing can alter it for the additional authenticator checks described in the issue.

Written by the indexing model from the issue text.