Fix/remove custom import paths

[VisargD]

Title: remove custom import paths

Description:

• Replace module imports in src/apis/postMethod.ts, src/apis/embeddings.ts, src/apis/generations.ts
• Remove use of NodeJS.fetch namespace as its removed in recent versions of @types/node

Motivation:

• To replace module imports: Module path maps are not resolved in emitted code microsoft/TypeScript#10866
• To avoid build errors due to fetch namespace unavailability: [node] Expose global ResponseInit; remove NodeJS.fetch namespace DefinitelyTyped/DefinitelyTyped#67341

Related Issues:

• Closes Remove custom path imports #21
• Closes Remove nodejs fetch namespace reference #22