Improve fetchOptions type definition for better autocomplete, type safety, Next.js extensions

Description:

currently, the fetchOptions type in Axios is defined as Record<string, any>, which limits IDE autocompletion and type safety. This issue proposes updating the type to RequestInit, the standard interface used by the Fetch API. Additionally, Next.js extends the RequestInit interface with custom options like cache and next, making this change more relevant for developers using Axios in Next.js projects.

Why this matters:

• Improved Autocompletion
  • using RequestInit provides developers with context-aware suggestions in IDEs.
• Type Safety
  • ensures stricter type checking and reduces runtime errors.
• Next.js Compatibility
  • highlights Next.js-specific extensions (next) that are compatible with the standard RequestInit. TypeScript Declaration Merging for RequestInit in Next.js (global.d.ts)
  • additionally, this modification prepares Axios to accommodate future fetch options introduced by Next.js.

Proposed Solution:

• Replace the current type definition of fetchOptions with RequestInit.
• Ensure compatibility with Next.js extensions by leveraging its extended interface.

Benefits:

• Aligns Axios with Fetch API standards.
• Enhances developer experience, especially for Next.js users.
• Provides a safer and more predictable API.