Generate Substrait extension YAML from UDFs

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

DataFusion has a default runtime inventory of scalar, higher-order, aggregate, and window functions, but there is no generated Substrait extension YAML (https://github.com/substrait-io/substrait/blob/main/text/simple_extensions_schema.yaml) that describes that function set.

Without generated extension YAML, Substrait function declarations can drift from the functions registered by a default DataFusion session, and downstream projects have no reusable DataFusion API for generating equivalent YAML for custom UDF registries.

### Describe the solution you'd like

Add a generator that emits Substrait function YAML from the same default `SessionStateDefaults` inventory used by DataFusion's function docs generator.

The generator should:

* expose a public API that accepts an explicit function inventory and generator config
* allow custom URNs, output paths, and overrides for signatures or return types that cannot be inferred from DataFusion `Signature` metadata
* include a binary with stdout, `--check`, and `--write` modes
* commit generated scalar, aggregate, and window YAML files under `datafusion/substrait/extensions/`
* fail if a default function cannot be emitted or explicitly overridden

### Describe alternatives you've considered

The main alternative is to maintain the Substrait YAML by hand. That would be more error-prone because the source of truth for available DataFusion functions is the runtime registry, not static YAML files.

### Additional context

_No response_