Improve performance of fields_for method with memoization

[saiqulhaq]

Description

This PR implements memoization to improve the performance of the fields_for method in the ActiveModel::Serializer::Fieldset class.

I found that this gem calls #fields_for method multiple times.

Motivation and Context

The current implementation of fields_for performs string manipulations and hash lookups on every call. By memoizing the results, we can significantly reduce redundant operations, especially when the method is called frequently with the same type.

How Has This Been Tested?

The existing test suite was run before and after the changes. All tests pass, and there's a noticeable performance improvement:

Without memoization: Finished in 2.639453s, 217.8482 runs/s, 377.7298 assertions/s.

With memoization: Finished in 2.465571s, 233.2117 runs/s, 404.3688 assertions/s.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.