FEAT Support for other LLM APIs in crescendo

[gh-ita]

Is your feature request related to a problem? Please describe.

I want to use crescendo but with llms other then the ones included in the implementations of the PromptChatTarget class necessary for crescendo

Describe the solution you'd like

I need PromptChatTarget class that supports gemini 1.5 flash api

Describe alternatives you've considered, if relevant

Additional context