key: add Nl80211KeyDefaultType and NL80211_KEY_DEFAULT_TYPES support

[cathay4t]

Problem:

The Nl80211KeyAttr enum was missing NL80211_KEY_DEFAULT_TYPES (kind 8),
preventing users from marking a group key as the default multicast key via
NL80211_CMD_SET_KEY.

Fix:

Add Nl80211KeyDefaultType enum mirroring kernel
enum nl80211_key_default_types (Unicast, Multicast).
Add DefaultTypes(Vec<Nl80211KeyDefaultType>) variant to Nl80211KeyAttr
with full encode/decode support.
Export the new type from the crate root.

Existing round-trip tests in tests/key.rs continue to pass.

[gemini-code-assist]

Code Review

This pull request adds support for Extended RSN Capabilities (RSNXE, element ID 244) using a u128 bitflags representation to handle variable-length capability fields. It also introduces support for the NL80211_KEY_DEFAULT_TYPES key attribute and its nested Nl80211KeyDefaultType sub-attributes (Unicast and Multicast). A compilation issue was identified in src/key.rs where .context() is incorrectly called on NlaBuffer items yielded by NlasIterator during parsing.

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