refactor for structured Mxc type

Cargo.lock

Cargo.toml

src/admin/media/commands.rs

src/api/client/media.rs

src/core/error/mod.rs

src/service/media/data.rs

src/service/media/mod.rs

src/service/media/preview.rs

src/service/media/remote.rs

src/service/media/thumbnail.rs