Speed up URL/tag generation via Configuration clone fast-path

[const-cloudinary]

Reduce per-asset/per-tag Configuration construction overhead and tighten a few hot paths exercised by URL building.

• Add Configuration::__clone() that deep-clones every section so cloning the global singleton produces a fully independent working copy without the cost of a JSON import/export round-trip.
• Use the clone fast-path in BaseAsset::fromParams, BaseTag::configuration (replaces a pre-existing TODO), and BaseTag::fromParamsDefaultConfig instead of new Configuration(...).
• Memoize BaseConfigSection::exportableKeys() per (class, sensitivity) to eliminate repeated ReflectionClass::getConstants() calls on every config serialization.
• Replace array_values(section)[0] with current(section) in the two jsonSerialize implementations.

Add tests/Unit/Configuration/ConfigurationCloneTest.php to lock in clone isolation across all configuration sections and across the asset/tag fast-paths, so a future regression to a shallow copy fails loudly.

Brief Summary of Changes

What does this PR address?

• GitHub issue (Add reference - #XX)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all of the tests pass.