[Go] Add simulated constrained generation support

[apascal07]

Describe the bug
This is currently what happens by default (output conformance messages are appended to the user request) but we should be checking for whether the model supports native constrained generation first.

[apascal07]

Louise, this is what you're doing first as part of the formatters work, I just wanted to break it out into its own issue because adding native constrained generation to the Gemini plugin is dependent on this.