[Go] generate configuration is a generic map when using DevUI model prompt view

[hugoaguirre]

Describe the bug
When using DevUI in the Model view, users can tune model configuration for the generate request call.

When tracing down the request configuration in Genkit plugins, we realized the request configuration comes as a map[string]any instead of the expected type (ai.GenerationCommonConfig) causing the request not to be properly configured in the plugin.

Here's where the bug resides (this is just for gemini plugin but other plugins might be affected):
https://github.com/firebase/genkit/blob/main/go/plugins/googleai/googleai.go#L325

To Reproduce
Use DevUI to send a request, configuration made in the view won't take effect on the request

Expected behavior
Genkit generate call should be configured based on user inputs

Screenshots
Create a debug print for the else branch

Execute the Generate call using DevUI and go back to the logs, the error will show