Feature/add websocket support

[sebastianbuechler]

Add WebSocket support

This feature adds basic WebSocket support to apidash. It is according to the sketches in #15, but the headers are not yet incorporated into the web socket connection. Including the body does not make sense for WebSockets if I'm not mistaken, so we should remove it as well.

My proposal for the query params and the headers: If they get changed, the connection should be dropped and the user needs to manually reconnect, as the server connection will change. Maybe the UI should be adapted for this.

Thus open points:

• Remove body tab
• Include query params into WebSocketManager
• Handle query param changes into an existing connection
• Include headers into WebSocketManager
• Handle header changes into an existing connection

Testing is a bit harder for WebSockets to begin with. Maybe inject an HTTP client manually to mock it?

Related Issues

• Closes Feature request: Websocket Support #15

Checklist

• I have gone through the contributing guide
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: please replace this line with details on why tests have not been included
• I need help with writing tests

[sebastianbuechler]

@ashitaprasad In contrast to #215 this works with the same request model and dedicated WebSocket states. This allows for simultaneous connections to WebSocket servers. Let me know if adaptions are needed or further clarification on the specs.

[animator]

@sebastianbuechler Kindly make this change #307