[Live] Expose RunConfig in `get_fast_api_app`

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
For the Live API, the RunConfig exposes key configurations such as VAD and Speech configuration.
I do not see any way of configuring the RunConfig from the outside, and it's declaration is hidden in the runner `run_live` / `AdkWebServer.run_agent_live`.

https://github.com/google/adk-python/blob/38ea749c9cec8e65f5e768f49fd2de79b5545571/src/google/adk/runners.py#L773C1-L773C43
https://github.com/google/adk-python/blob/38ea749c9cec8e65f5e768f49fd2de79b5545571/src/google/adk/cli/adk_web_server.py#L1516C1-L1520C19

**Describe the solution you'd like**
Expose it to the `get_fast_api_app` method, or make it easier to override a single api.

**Describe alternatives you've considered**
I tried copy pasting the `/run_live` API outside but there are dependencies with the `AdkWebServer` class ( `self.session_service` and `self.get_runner_async` ).
This makes it difficult to override the behaviour of the `/run_live` api.

**Additional context**
none.