Expose writer functions

[zcourts]

Would you be open to some refactor of the functions in

https://github.com/databendlabs/opensrv/blob/main/mysql/src/writers.rs#L63

ErrorKind doesn't have all the MySQL error codes, for example ER_NO_SUCH_DB

Instead of doing a PR for each one that pops up I was thinking the functions could be refactored to accept the u16 and the PacketWriter made public instead of (create) so the fns can be called outside the crate and have a ref to pass in.