refactor(core): unify auth error handling and update docs

• Add shared errNilRequest used by all auth types for nil request errors.
• Simplify and standardize error messages for API key and query auth.
• Update README feature list, examples, and Go version requirement.
• Rename defaultHeaders to headers in New and adjust comments.
• Clarify transport tweaking logic in build and improve comments.
• Minor formatting and comment cleanup throughout client and auth code.

Release v1.0.1

- Update README documentation to use `Do` instead of `Send` for streaming responses.
- Bump the library version constant to v1.0.1.
- No breaking changes.

feat: initial release of httpx v1.0.0

Go HTTP client with retries (backoff, Retry-After), middleware,
detailed HTTPError, and multipart uploads via FileReader.
Includes CI workflow, README, and MIT license.