fix(parquet/internal/encoding): Fix typed dictionary encoding

[MasslessParticle]

Rationale for this change

The typed dict encoder tries to cast memory.Allocator to TypedMemoTable. This results in a panic.

What changes are included in this PR?

This PR changes mem to memo in the encoder so the MemoTable is the variable cast to TypedMemoTable.

Are these changes tested?

I've tested this locally and panics no longer happen 🎉

[zeroshade]

Thanks for finding this!

There wasn't any test cases which hit this before?

Would you be able to add a test case that hits this to ensure we don't have any regressions in the future?

[zeroshade]

@MasslessParticle any luck on adding a test for this?

[MasslessParticle]

@zeroshade Sorry, I got distracted last week. I've added a very basic test. I've only just started learning about the arrow ecosystem so it doesn't capture any other context than "we can call this without a panic or error."

Let me know what you think.

[zeroshade]

Thanks for this!