[flight] make the enforce_schema function public

[ccciudatu]

Expose the (slightly enhanced) enforce_schema function to downstream application code.

[phillipleblanc]

Thanks!

[ccciudatu]

FWIW, I think this functionality is generic enough to be useful in a far broader context, beyond flight and even table-providers. My guess is there are variations of this already rewritten again and again across various projects.
@alamb do you think this would be a good fit for upstream crates like arrow-cast or datafusion-common?

[alamb]

FWIW, I think this functionality is generic enough to be useful in a far broader context, beyond flight and even table-providers. My guess is there are variations of this already rewritten again and again across various projects. @alamb do you think this would be a good fit for upstream crates like arrow-cast or datafusion-common?

I took a quick look at what enforce_schema does and it looks to me like it does something called "schema evolution" in other contexts

This code in DataFusion seems to do the same thing (map a batch schema to a target schema, filling in null columns / casting types when needed):
https://docs.rs/datafusion/latest/datafusion/datasource/schema_adapter/struct.DefaultSchemaAdapterFactory.html

[alamb]

The docs were a bit lackluster so I added some more here: apache/datafusion#13063