feat: WIP relationships and threads

Cargo.lock

Cargo.toml

Cross.toml

src/api/client_server/context.rs

src/api/client_server/media.rs

src/api/client_server/message.rs

src/api/client_server/mod.rs

src/api/client_server/relations.rs

src/api/client_server/threads.rs

src/api/client_server/unversioned.rs

src/api/server_server.rs

src/database/key_value/rooms/mod.rs

src/database/key_value/rooms/pdu_metadata.rs

src/database/key_value/rooms/threads.rs

src/database/key_value/rooms/timeline.rs

src/database/mod.rs

src/main.rs

src/service/media/mod.rs

src/service/mod.rs

src/service/pdu.rs