genai: add samples for caching

[eliben]

No description provided.

[eliben]

Note the userContent function, it makes it makes it much nicer to create simple contents, e.g.

instead of:

x = *genai.Content{Role: "user", Parts: []genai.Part{genai.Text{"foo"}}}

one can write:

x = userContent(genai.Text{"foo"}}

In lanchgaingo there's an exported function that does this, and it's very convenient for users.

I'm wondering whether we should have such a function exported from the SDK as well

[jba]

Sure, maybe NewUserContent.

[eliben]

Sure, maybe NewUserContent.

OK, going to submit this as is and add that function in a separate PR where I'll also try to find locations where to apply it