Expose the inline Python UDF payload prefixes publicly #1817

Is your feature request related to a problem or challenge?

Ballista's Python client wraps datafusion-python's PythonLogicalCodec and needs to recognise inline Python UDF payloads so it can route them to that codec. The family prefixes it matches on are pub(crate) in crates/core/src/codec.rs:

pub(crate) const PY_SCALAR_UDF_FAMILY: &[u8] = b"DFPYUDF";
pub(crate) const PY_AGG_UDF_FAMILY: &[u8] = b"DFPYUDA";
pub(crate) const PY_WINDOW_UDF_FAMILY: &[u8] = b"DFPYUDW";

So Ballista has to keep its own copies (apache/datafusion-ballista#2619), which will silently break if the wire format changes here.

Describe the solution you'd like

Make these constants pub, or expose a public helper that reports whether a buffer is an inline Python UDF payload (something like the existing payload-detection functions, made public).

Describe alternatives you've considered

Keep copying the constants downstream and track wire-format changes by hand.

Additional context

Downstream use: apache/datafusion-ballista#2619 (comment)