enhance: support credential override

Enable users to set credential overrides on `run`.

e.g.

```typescript
const g = new gptscript.GPTScript();
try {
  const run = await g.run('./test.gpt', {
    disableCache: true,
    credentialOverride: "sys.openai:OPENAI_API_KEY",
  });
  console.log(await run.text());
} catch (e) {
  console.error(e);
}
g.close();
```

```yaml
tools: github.com/gptscript-ai/dalle-image-generation

You are an expert in image generation. Please generate a lion standing proudly in the savannah.
```

Signed-off-by: Nick Hale <[email protected]>

Author: njhale

src/gptscript.ts

@@ -37,6 +37,7 @@ export interface RunOpts {
 	chatState?: string
 	confirm?: boolean
 	prompt?: boolean
+    credentialOverride?: string
 	env?: string[]
 
 	APIKey?: string