feat: minimal fmt::Debug impl for STUN Attributes

[thomaseizinger]

When the debug log level is activated, str0m logs each STUN request that it sends. This is useful but in its current configuration very noise, see:

| 2024-01-25T01:08:01.612757Z DEBUG str0m::ice_::agent: Send STUN request: 172.18.0.3:50261 -> 172.20.0.3:52550 StunMessage { method: Binding, class: Request, attrs: Attributes { username: Some("gpzm:qUls"), message_integrity: None, error_code: None, realm: None, nonce: None, xor_mapped_address: None, software: None, fingerprint: None, priority: Some(1862270719), use_candidate: false, ice_controlled: None, ice_controlling: Some(7816118197453770412), network_cost: None }, integrity_len: 0 }

This PR only logs the fields that are actually set which will transform the above into:

| 2024-01-25T01:08:01.612757Z DEBUG str0m::ice_::agent: Send STUN request: 172.18.0.3:50261 -> 172.20.0.3:52550 StunMessage { method: Binding, class: Request, attrs: Attributes { username: "gpzm:qUls", priority: 1862270719, ice_controlling: 7816118197453770412 }, integrity_len: 0 }

[algesten]

Thanks!