`mediaResolution` parameter missing from GenerationConfig

[ArturLog]

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