Connect to Unix Domain sockets by abstract names

[sivizius]

• refactor: restructure and reuse code: Not exactly necessary but it helped implementing the following changes.
• feat(firecracker,unix): use OsString to allow abstract names: Previously, The authority-part of the URI was parsed as hexadecimal into a Vec<u8> but those octets parsed as UTF8 string before converted to PathBuf. However, The inner type of Path/PathBuf is OsStr/OsString which actually allows NULL bytes (\0). tokio checks the first octet of the path in UnixStream::connect: https://docs.rs/tokio/latest/src/tokio/net/unix/stream.rs.html#72-99 to determine whether a pathname or an abstract name is encoded. I thus changed the parsing to OsString directly to allow encoding abstract names in the URI.
• feat(firecracker,unix): unix::SocketAddr methods for Uri extensions: Those are rather convenient methods to construct and parse URIs with abstract names.