unify env var/default resolution into shared primitives

Part of #23740.

`StreamingOptions`'s `_opt()` and `StreamingExecutor`'s `_make_default_factory()` are
independently written, near-identical implementations of "explicit value, then env
var(s), then default". An inline block in `ConfigOptions.from_polars_engine` and
`MemoryResourceConfig()`'s own construction are two more independent copies of "explicit
value, then build a nested object if enabled, then a fallback", for the memory resource
config specifically. Replace both groups with one shared function each, used everywhere a
field of that shape is declared, regardless of which dataclass declares it.