Add caching mechanism to DefaultsOnlyConfigurationService for default values

## Feature Description
To improve performance and reduce redundant lookups on the global configuration registry, we can introduce a simple Map-based caching mechanism inside `DefaultsOnlyConfigurationService`.

## Proposed Changes
- Added a private `_defaultValueCache = new Map()` property.
- Created a `_getCachedDefaultValue` helper method to store and retrieve default values efficiently.
- Updated `getConfig`, `inspectConfig`, and `getExperimentBasedConfig` to utilize the cached default values.