Add --timeout flag to configure API request timeout

## Problem

The CLI uses a hardcoded 30-second timeout for all API requests (defined in `api.py` as `DEFAULT_TIMEOUT = 30.0`). Users on slow connections or processing large documents may need more time.

## Solution

Add a global `--timeout` flag to override the default timeout:

```bash
jina read https://large-pdf-url.com --timeout 60
jina search "complex query" --timeout 45
```

## Use Case

Users with slow network connections or when fetching large documents (PDFs, long web pages) that take longer than 30 seconds to process.