Add 'Seed' parameter to GenerationConfig model

[taurit]

Is your feature request related to a problem? Please describe.
By default, the responses from the Generative AI APIs are non-deterministic and change with each request. This poses a problem, for instance, in automated tests, which become flaky - sometimes they pass and sometimes fail without any changes in code.

Describe the solution you'd like
I want to be able to seed the random seed value in the client. The API already supports it. It should be enough to add the Seed property to the GenerationConfig model:

 public class GenerationConfig
 {
   // ... existing properties

   [JsonProperty("seed")]
   public int Seed { get; set; }
 }

Describe alternatives you've considered
Persisting/caching responses on the application side, but it's hardly an alternative, just a workaround.

Additional context
N/A