enhance: support credential override

Enable users to set credential overrides on `run`.

```python
gptscript = GPTScript()

run = gptscript.run('./test.gpt',
opts=Options(credentialOverride='sys.openai:OPENAI_API_KEY')
output = await run.text()
print(output)

gptscript.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

gptscript/opts.py

@@ -30,6 +30,7 @@ def __init__(self,
                  chatState: str = "",
                  confirm: bool = False,
                  prompt: bool = False,
+                 credentialOverride: str = "",
                  env: list[str] = None,
                  apiKey: str = "",
                  baseURL: str = "",
@@ -43,4 +44,5 @@ def __init__(self,
         self.chatState = chatState
         self.confirm = confirm
         self.prompt = prompt
+        self.credentialOverride = credentialOverride
         self.env = env