Upgrade to datafusion 50 (#171)

* Upgrade to datafusion 50

Upgrade to datafusion 50 & make associated code changes.  Also upgrade
to arrow 56, which matches what datafusion 50 depends on internally.

* Upgrade to Rust 1.86.0

Upgrade to Rust 1.86.0 from 1.85.1

---------

Co-authored-by: Ning Sun <[email protected]>

Author: mjgarton

.github/workflows/ci.yml

@@ -81,6 +81,6 @@ jobs:
     - uses: actions/checkout@v4
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: "1.85.1"
+        toolchain: "1.86.0"
         override: true
     - run: cargo build --all-features