Enhance code re-usability

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

It seems that it could not be possible to manage the upgrade event in custom way, without re-writing the entire logic.
The cause seems to be the Symbols used to store headers and socketRef into the `rawReq` object (`kWs, kWsHead`)

### Motivation

Code re-usability.
In my case I have to add custom headers on HTTP Upgrade response #204 (`ws` library under the hood, supports it).

### Example

Replace Symbol usage with simple string keys or expose some properties to manage the key name attribute.