Add a typed SessionConfigExt setter for ballista.execution.mode #2729

Is your feature request related to a problem or challenge?

To use pipelined execution (#2652) from Rust today, you set the mode by its string key:

let config = SessionConfig::new_with_ballista()
    .set_str("ballista.execution.mode", "pipelined");

Most other Ballista session settings have a typed helper on SessionConfigExt, such as with_ballista_adaptive_query_planner(bool). The execution mode doesn't have one, so you have to know the key and its string values. This came up in review of the pipelined execution user guide (#2713 (comment)).

Describe the solution you'd like

Add a typed setter and getter to SessionConfigExt, following the existing with_ballista_* convention:

fn with_ballista_execution_mode(self, mode: ExecutionMode) -> Self;
fn ballista_execution_mode(&self) -> ExecutionMode;

so enabling the mode becomes:

let config = SessionConfig::new_with_ballista()
    .with_ballista_execution_mode(ExecutionMode::Pipelined);

ExecutionMode would need to be reachable from the client crate's prelude or re-exports. The Rust snippet in docs/source/user-guide/pipelined-execution.md should then switch to the helper.

Describe alternatives you've considered

A dedicated constructor, SessionConfig::new_with_ballista_pipelined(). That would add a constructor per mode, and it wouldn't compose with configs built some other way. A setter matches how the other settings are exposed.

Additional context

The getter could also replace the scattered ballista_config().execution_mode() calls that parse the string.