Expose SessionContext.add_optimizer_rule for Python-defined logical optimizer rules

## Background

`SessionContext::add_optimizer_rule` registers an `Arc` that runs during logical plan optimization. The Python bindings only expose `remove_optimizer_rule`, never the additive side. PR #1557 ultimately landed `add_physical_optimizer_rule` for the physical pipeline via FFI but did not address the logical pipeline.

## Upstream signature

```rust
pub fn add_optimizer_rule(&self, optimizer_rule: Arc)
```

## User value

Lets users add domain-specific logical rewrites (predicate normalization, redundant join elimination, scan pruning informed by external metadata) without forking DataFusion. Complements the existing `remove_optimizer_rule` to round out the surface.

## Why deferred

Blocked upstream. As documented in PR #1557, DataFusion does not currently expose an FFI bridge for the logical `OptimizerRule` / `AnalyzerRule` traits, and there are no Python constructors for `LogicalPlan` node variants -- a pure-Python rule could observe plans but not transform them. The PR #1557 commit history shows an initial attempt at a Python-defined logical rule that was abandoned for this reason. This is filed for tracking; it should be revisited once upstream lands an `FFI_OptimizerRule` (mirroring `FFI_PhysicalOptimizerRule`).