Allow me to pass custom sessionId and state json on the ADK Web

**Is your feature request related to a problem? Please describe.**
I'm frustrated that while the ADK Web allows basic testing, it does not allow me to pick my own sessionId, nor to pass a custom state to the user.

The google adk documentation shows an example createSession endpoint call:
{URL}/apps/{appname}/users/{username}/sessions/{sessionId}
This request also has a JSON body that allows us to pass custom session key/value pairs, that can be used for more complex agents. e.g. :
{
"state": {
"test_number": 95
},
"userdata": {
"user_name": "miguel"
}
}

In the request above I am able to pick the agent (which google adk already allows me to do), a user, and a sessionId.
In adk web I am unaware what user it picks and it is sending its own sessionId. I can also not pass the custom state, so if my tools or the agent system use them, I cant test.

**Describe the solution you'd like**
I feel the solution is simple, when creating a new chat, allow me to either just have it create it randomly as it does, or do it passing myself the values I need (user, sessionId, state json).
It would be a quick solution that would go very far in allowing me to test complex agents.

**Describe alternatives you've considered**
I have to either use postman or curl which is not ideal to visualize agentic workflows, or a custom frontend.