[Go] Allow Dynamic Prompts on MCP

Current registered prompts do no change based on input and they keep preserved as "snapshots".

Expand Prompt functionallity to allow dynamic content prompts, this will result in a new API:
```go
func (c *GenkitMCPClient) GetDynamicPrompt(ctx context.Context, g *genkit.Genkit, name string) (ai.Prompt, error)
```


