Allow passing descriptor_set as bytes instead of a file path

Currently, buffa-build allows passing a descriptor_set to use instead of proto files

https://github.com/anthropics/buffa/blob/ccc9adddb51274a45a0155d40514137504904140/buffa-build/src/lib.rs#L1640

This is then just read into bytes

https://github.com/anthropics/buffa/blob/ccc9adddb51274a45a0155d40514137504904140/buffa-build/src/lib.rs#L1772

It would be great if bytes could be passed directly for when the build script can prepare them, for example using protox. Perhaps changing `Precompiled` to hold a `Vec` and have the parameter accept either a sealed trait that is implemented for both `Path` and `&[u8]`, or if breaking is ok, just accept the slice only since reading a path (as buffa-build currently does) wouldn't be that hard for a user.