[Extensions] Handle unknown extensions

Nobody has claimed this yet.

Extensions unknown to the client should be passed through as CBOR. Maybe the trait itself could expose the expected input key, and if no extension is found, the extension data can be passed as-is?
```rust
trait ClientExtensionParsing where G: TryFrom, C: TryFrom {
const MAKE_INPUT_KEY: &str;
const GET_INPUT_KEY: &str;
}
trait ClientExtension: ClientExtensionProcessing + ClientExtensionParsing
```

Ideally, if we fail to parse input for an extension, we should ignore the individual extension - but continue with the rest of the request. Maybe the trait for each extension could be generic over its input type, and perform parsing, with a Result so the caller can handle these errors appropriately?

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

Start by tracing the ClientExtensionParsing and ClientExtension traits and the existing extension processing path; no files or tests are named in the issue. Define the behavior for unknown extensions and input parse failures, including preserving unknown data as CBOR, ignoring only the failing extension, and continuing the request.

Written by the indexing model from the issue text.