feat(openapi-fetch): allow usage of custom Request class

[tobiasdcl]

Changes

• allows to specify a custom Request class
  • for more information around the motivation see #1563
  • build on the foundation of #1593

How to Review

• I removed the pre-existing CustomRequest class and inlined the business logic from the constructor
• added a unit test

Checklist

• [ ] Unit tests updated
• [ ] docs/ updated (if necessary)
• [ ] pnpm run update:examples run (only applicable for openapi-typescript)