Minor: Add getter for logical optimizer rules

[maronavenue]

Which issue does this PR close?

N/A

Rationale for this change

My team and I have been exploring Datafusion's potential to federate our custom sources performantly. Part of that is tuning optimizers depending on our needs. Noticed that there isn't a convenience method in SessionState yet that returns logical optimizer rules compared to its physical counterpart. Would be great for our POC work to quickly inspect them outside (and/or alongside) debugger or observer.

What changes are included in this PR?

Adds a getter method optimizers() in SessionState in parity to the physical_optimizers() counterpart.

Are these changes tested?

Yes, ran new and existing tests. Also manually tested from our existing POC code.

Are there any user-facing changes?

Examples:

let state = SessionStateBuilder::new()
    .with_optimizer_rules(vec![
        Arc::new(UnwrapCastInComparison::new()),
        Arc::new(SimplifyExpressions::new()),
        Arc::new(OptimizeProjections::new()),
        Arc::new(PushDownFilter::new()),
    ])
    .build();

let optimizer_rules = state.optimizers();
for rule in optimizer_rules {
    println!("Optimizer: {}", rule.name());
}

[maronavenue]

This is my first time contributing. Please let me know if I missed any. Tagging for CI, @alamb @andygrove

[Weijun-H]

LGTM! Thanks @maronavenue

[Weijun-H]

nit

Suggested change

	// test building sessions with supplied rules
	struct DummyRule {}
	impl OptimizerRule for DummyRule {
	fn name(&self) -> &str {
	"dummy_rule"
	}
	}
	// test building sessions with supplied rules

[maronavenue]

@Weijun-H - Thanks for having a look! Applied your feedback.

bba3fe5

[Weijun-H]

nit

Suggested change

	struct DummyRule {}
	impl OptimizerRule for DummyRule {
	fn name(&self) -> &str {
	"dummy_rule"
	}
	}

[Weijun-H]

Thanks @maronavenue for contribution and @Dandandan for review