Use credentials for API keys instead of plain text strings

[koettert]

All node that use/require an API key should support a credentials flow variable which can be used via the get_credentials(identifier: str)→ Credential method.

Ideally we wait until the Python framework supports credentials (AP-21962).

The dropdown box for credentials variables can be created like this:

@knext.parameter_group(label="Credentials")
class CredentialsSettings:
    credentials_param = knext.StringParameter(
        label="Credentials parameter",
        description="Credentials parameter name for accessing Google Search API key.",
        choices=lambda a: knext.DialogCreationContext.get_credential_names(a),
    )

For details see the src/agents/base.py of the knime-python-llm repository.