Upgrade to datafusion 51 & arrow 57 (#240)

* Upgrade to datafusion 51 & arrow 57

Upgrade datafusion & arrow dependencies and make required code changes.

Note that although `cargo test` completes normally, the duckdb example
does not yet work, due to the dependency on arrow 56.

* Increase MSRV to 1.88.0

Increase MSRV to 1.88.0 to match datafusion 51.

* Attempt to fix datatype error

Integration tests are failing after upgrading datafusion & arrow:

Function 'current_schemas' returned value of type 'List(Field { data_type: Utf8, nullable: true })' while the following type was promised at planning time and expected: 'List(Field { name: "schema", data_type: Utf8 })'.

Attempt to fix this my making our `current_schema` UDF declare the same
return type that is actually being returned.

This seems to be _a_ fix for the error, but I'm not certain it's the
_right_ fix.

Author: mjgarton

.github/workflows/ci.yml

@@ -78,6 +78,6 @@ jobs:
     - uses: actions/checkout@v4
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: "1.86.0"
+        toolchain: "1.88.0"
         override: true
     - run: cargo build --all-features