[feat] Configuring request header before specifying the method

## Configuring request header before specifying the method, or creating agent instances that keep memory of what we configured him with.

How I would like Superagent to be:

```javascript
const superagent = require('superagent')

const agent = superagent(app).createAgent()

agent.set({
accessToken: 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c',
refreshToken: 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c'
})

agent.get('/currentUser')
```

## Checklist

- [X] I have searched through GitHub issues for similar issues.
- [X] I have completely read through the README and documentation.

No implementation files or tests are named. Start by tracing the existing request-construction and header-setting API around the documented Superagent usage, then compare it with the proposed createAgent(), agent.set(), and agent.get() flow. Done means the persistent-header behavior is agreed, implemented, documented, and covered by tests.

Written by the indexing model from the issue text.