Update default model from `Gemini 1.0 Pro` to newer `Gemini 1.5 Pro`

[taurit]

Is your feature request related to a problem? Please describe. I'm a developer who wanted to use the library. It took me a while to understand that, by default, the Client doesn't use the most recent model (1.5) but an older one (1.0).

Describe the solution you'd like Currently, in GoogleGeminiConfig, the default model is gemini-pro, which is tied to Gemini 1.0. I think this tag will continue to point to 1.0. Version 1.5 was released in February 2024, quite a long time ago, and the tag didn't change.

I suggest using the state-of-the-art model as the default in the library:

-public string TextBaseUrl { get; set; } = "https://generativelanguage.googleapis.com/v1/models/gemini-pro";
+public string TextBaseUrl { get; set; } = "https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-pro-latest";

Describe alternatives you've considered As a library user, I know I can already set the model URL myself.

Additional context Some similar libraries like Betalgo.OpenAI use enums (and not strings) to help developers easily choose the model without looking for strings in the documentation:

var completionResult = await openAiService.ChatCompletion.CreateCompletion(new ChatCompletionCreateRequest
{
    Messages = new List<ChatMessage> { ChatMessage.FromUser("Who won the world series in 2020?"), },
    Model = Models.Gpt_4o, // this is convenient
});

This has the downside of maintaining the enum list, but maybe such enum would be beneficial for ease-of-use here too?