[Rust][get_options] Use custom merge for 8-10X perf improvement

[juharris]

Implement custom configuration merging to avoid big copies instead of using the config library.

From before some changes, so it should be even slightly better now:

Scenario	Config Crate	Custom Merge	Speedup
Single feature (get_options)	254 µs	30 µs	8.5x
Single feature (get_all)	284 µs	26 µs	10.9x
Multiple features (get_options)	680 µs	87 µs	7.8x
Multiple features (get_all)	761 µs	78 µs	9.8x
Cache hit	-	3.3 µs	Same

Config crate: ~254 µs for single feature, ~680 µs for multiple features
Custom merge: ~26-30 µs for single feature, ~54 µs for multiple features

[tavianator]

Suggested change

	if let Some(_cache_options) = cache_options {
	if cache_options.is_some() {

[juharris]

Thanks. Done in #161