feat(table): export StructLike and GetPartitionRecord

[twuebi]

PartitionRecord and GetPartitionRecord are the only way to go from a DataFile to something PartitionToPath accepts. Keeping them unexported forces external consumers to reimplement the same conversion.

Also deduplicates the manual record construction in the pos delete writer.

[zeroshade]

Doesn't PartitionToPath take an interface? Could we get away with just exporting GetPartitionRecord rather than also exporting the PartitionRecord type directly? It just feels like exporting an implementation detail to me.

[twuebi]

It does take a structLike which is also unexported, what should GetPartitionRecord return then? Thought that returning an unexported type may be a bit awkward