`mediaResolution` parameter missing from GenerationConfig

**Description**
I'm trying to use the `mediaResolution` parameter as described in the Vertex AI REST API documentation, but I’ve noticed that this library does not currently support this field. Specifically, the `GenerationConfig` does not include the `mediaResolution` parameter, which makes it impossible to fully utilize the API features through the Node.js client.

**Suggested fix**
Please update the `GenerationConfig` type definition in the library to match the actual REST API capabilities.

**Additional context**
Here’s the relevant documentation for the `mediaResolution` parameter:
https://ai.google.dev/api/generate-content#MediaResolution