Get topic from discovery data

[st9540808]

SPDP and SEDP are key components of the DDS discovery process. These acronyms stand for Simple Participant Discovery Protocol (SPDP) and Simple Endpoint Discovery Protocol (SEDP), respectively.

In short, SPDP is for discovering DomainParticipants (corresponding to ROS 2 node), and SEDP is for discovering the endpoints, such as DataReaders and DataWriters (correspond to ROS 2 subscriber and publisher) within those DomainParticipants.

Example debug print for builtin_data

[src/rtps.rs:96] builtin_data = DiscoveredReaderData {
    reader_proxy: ReaderProxy {
        remote_reader_guid: GUID {[1, 16, 0, 175, 110, 183, 135, 45, 5, 83, 222, 18] EntityId {[0, 0, 20] EntityKind::READER_NO_KEY_USER_DEFINED}},
        expects_inline_qos: false,
        unicast_locator_list: [],
        multicast_locator_list: [],
    },
    subscription_topic_data: SubscriptionBuiltinTopicData {
        key: GUID {[1, 16, 0, 175, 110, 183, 135, 45, 5, 83, 222, 18] EntityId {[0, 0, 20] EntityKind::READER_NO_KEY_USER_DEFINED}},
        participant_key: None,
        topic_name: "rt/clock",
        type_name: "rosgraph_msgs::msg::dds_::Clock_",
        durability: None,
        deadline: None,
        latency_budget: None,
        liveliness: None,
        reliability: None,
        ownership: None,
        destination_order: None,
        time_based_filter: None,
        presentation: None,
        lifespan: None,
        service_instance_name: None,
        related_datawriter_key: None,
        topic_aliases: None,
    },
    content_filter: None,
}
[src/rtps.rs:96] builtin_data = DiscoveredReaderData {
    reader_proxy: ReaderProxy {
        remote_reader_guid: GUID {[1, 16, 0, 175, 110, 183, 135, 45, 5, 83, 222, 18] EntityId {[0, 0, 21] EntityKind::READER_NO_KEY_USER_DEFINED}},
        expects_inline_qos: false,
        unicast_locator_list: [],
        multicast_locator_list: [],
    },
    subscription_topic_data: SubscriptionBuiltinTopicData {
        key: GUID {[1, 16, 0, 175, 110, 183, 135, 45, 5, 83, 222, 18] EntityId {[0, 0, 21] EntityKind::READER_NO_KEY_USER_DEFINED}},
        participant_key: None,
        topic_name: "rt/parameter_events",
        type_name: "rcl_interfaces::msg::dds_::ParameterEvent_",
        durability: None,
        deadline: None,
        latency_budget: None,
        liveliness: None,
        reliability: Some(
            Reliable {
                max_blocking_time: Duration {
                    seconds: 2147483647,
                    fraction: 4294967295,
                },
            },
        ),
        ownership: None,
        destination_order: None,
        time_based_filter: None,
        presentation: None,
        lifespan: None,
        service_instance_name: None,
        related_datawriter_key: None,
        topic_aliases: None,
    },
    content_filter: None,
}