Can't serialize example `ExecutionPlan` to substrait

### Is your feature request related to a problem or challenge?

While working on an example for serializing substrait plans (see https://github.com/apache/arrow-datafusion/pull/9260 PR), I found I could not write an example for serializing an execution plan. The feature is still not complete enough

### Describe the solution you'd like

What I would like:

I would like to add the example (or something like it) to `datafusion/substrait/src/lib.rs` and have
it work.

//! # Example: Serializing [`ExecutionPlan`]s

//!

//! This functionality is still under development and only works for a small subset of plans

//!

//! ```

//! # use datafusion::prelude::*;

//! # use std::collections::HashMap;

//! # use std::sync::Arc;

//! # use datafusion::error::Result;

//! # use datafusion::arrow::array::Int32Array;

//! # use datafusion::arrow::record_batch::RecordBatch;

//! # use datafusion_substrait::physical_plan;

//! # #[tokio::main(flavor = "current_thread")]

//! # async fn main() -> Result<()>{

//! // Create a plan that scans table 't'

//!  let ctx = SessionContext::new();

//!  let batch = RecordBatch::try_from_iter(vec![("x", Arc::new(Int32Array::from(vec![42])) as _)])?;

//!  ctx.register_batch("t", batch)?;

//!  let df = ctx.sql("SELECT x from t").await?;

//!  let physical_plan = df.create_physical_plan().await?;

//!

//!  // Convert the plan into a substrait (protobuf) Rel

//!  let mut extension_info= (vec![], HashMap::new());

//!  let substrait_plan = physical_plan::producer::to_substrait_rel(physical_plan.as_ref(), &mut extension_info)?;

//!

//!  // Decode bytes from somewhere (over network, etc.) back to ExecutionPlan

//!  let physical_round_trip = physical_plan::consumer::from_substrait_rel(

//!     &ctx, &substrait_plan, &HashMap::new()

//!  ).await?;

//!  assert_eq!(format!("{:?}", physical_plan), format!("{:?}", physical_round_trip));

//! # Ok(())

//! # }

When you run this test today you get an error about "mem provider not implemented" or something like that

### Describe alternatives you've considered

_No response_

### Additional context

I think making this work would be a matter of implementing serialization of MemTable / MemExec perhaps

Start with the example proposed in datafusion/substrait/src/lib.rs and run it to reproduce the mem provider error. Read the physical_plan producer and consumer paths, focusing on the MemTable/MemExec serialization mentioned in the issue. Done means the example can serialize and deserialize the execution plan and its round-trip assertion succeeds.

Written by the indexing model from the issue text.