Custom Metadata Support In `RunConfig`

Please, tell me will customMetadata support in `RunConfig` be added in Google Java ADK like in Python? [Python ADK reference](https://github.com/google/adk-python/blob/main/src/google/adk/agents/run_config.py)

Case: we have additional information injection (context info) before the start of runner, but providing it in the content is a wrong approach, cause ADK framework with LLM under the hood parse request and final JSON contain only a message provided by user. It doesn't includes .

An example of injected content in prompt (the content will be run from `Runner`):
```json
{
"user_request" : "Return a JSON you've been prompted.",
"information_about_user" : {
"name" : "Valerii",
"surname" : "Fedorovych"
},
"current_time" : "2026-02-17T10:37:53.706185753+02:00"
}
```

Actual JSON processing by LLM:
```json
{
"request": {
"agent": "main_coordinator",
"user_id": "user_777_investor",
"session_id": "session_finance_tracker_001",
"message": "Return a JSON you've been prompted.",
"context": {
"previous_messages": [
],
"user_profile": {
"name": null,
"goals": [
"real_estate_investment"
]
}
},
"timestamp": "2024-05-22T14:25:00Z"
}
}
```

Looks like a `custom_metadata` used in `run_config` in Python ADK can resolve the issue.