Implement FFI_Session (#19223)

## Which issue does this PR close?

Addresses part of #18671 but
does not close it.

## Rationale for this change

One of the reasons we are unable to remove `SessionContext` from the FFI
crate is that we rely on building a session in the `TableProvider::scan`
function. This is because our current implementation uses
`FFI_SessionConfig` and passes around a hashmap of String->String values
of config options. Then in the other side of the FFI boundary we build a
temporary session. This is not ideal because those table provider cannot
utilize the actual current session.

By implementing a FFI safe version of the `Session` trait we can
overcome this limitation.

## What changes are included in this PR?

- Implement `FFI_Session`

This PR does _not_ use these structures in the current code. That is
coming as part of a later PR in an effort to keep the size of the PRs
small for effective code review.

## Are these changes tested?

Unit tests are added. Coverage report:

<img width="531" height="185" alt="Screenshot 2025-12-09 at 8 35 56 AM"
src="https://github.com/user-attachments/assets/1b6de980-e048-49cf-a8fe-961d5e902e06"
/>

## Are there any user-facing changes?

No.

Author: timsaucer

Cargo.lock

@@ -58,6 +58,7 @@ datafusion-physical-expr-common = { workspace = true }
 datafusion-physical-plan = { workspace = true }
 datafusion-proto = { workspace = true }
 datafusion-proto-common = { workspace = true }
+datafusion-session = { workspace = true }
 futures = { workspace = true }
 log = { workspace = true }
 prost = { workspace = true }

datafusion/ffi/Cargo.toml

@@ -28,7 +28,7 @@ use datafusion_expr::{
     AggregateUDF, AggregateUDFImpl, ScalarUDF, ScalarUDFImpl, WindowUDF, WindowUDFImpl,
 };
 
-use crate::session_config::FFI_SessionConfig;
+use crate::session::config::FFI_SessionConfig;
 use crate::udaf::FFI_AggregateUDF;
 use crate::udf::FFI_ScalarUDF;
 use crate::udwf::FFI_WindowUDF;

datafusion/ffi/src/execution/task_ctx.rs

@@ -37,7 +37,7 @@ pub mod plan_properties;
 pub mod proto;
 pub mod record_batch_stream;
 pub mod schema_provider;
-pub mod session_config;
+pub mod session;
 pub mod table_provider;
 pub mod table_source;
 pub mod udaf;