This commit enhances security and configurability by enabling dynamic runtime
resolution of HTTP Basic Authentication credentials for WMS layers. Credentials
can now be injected from environment variables, reducing the need to hardcode
sensitive values. This improves code maintainability, supports secure multi-
environment deployments, and simplifies testing through dynamic configuration.

1. **Dynamic Environment Parametrization**:
   - Introduced `GeoWebCacheEnvironment#isAllowEnvParametrization()` to replace
     the static `ALLOW_ENV_PARAMETRIZATION` field, allowing runtime toggling.

2. **Environment Variable Resolution Refactor**:
   - Replaced direct static field checks with method calls.
   - Updated `resolveValue()` and related methods to use environment variables
     dynamically.

3. **WMS Credentials Management Update**:
   - Added `getResolvedHttpUsername()` and `getResolvedHttpPassword()` in
     `WMSHttpHelper`.
   - Created `setGeoWebCacheEnvironment()` for dependency injection.

4. **Testing Enhancements**:
   - Integrated the `system-rules` library for environment variable manipulation.
   - Added tests to cover default, custom, and parameterized credentials.

5. **Code Improvements**:
   - Replaced unsafe casts in `resolveValue()`.
   - Improved exception handling by switching from `Throwable` to
     `RuntimeException`.
   - Added better logging and documentation for credential handling.