Ensure SecretStr is cast to str on load for model clients (#5947)

Currently we have SecretStr type for model clients to promote security
best practices.

- when we dump_component, keys are serialized  as SecreteStr ..
- when we load_component ... SecreteStr type is passed to the client in
the api_key field. This i causes the type problems as the clients expect
a string type.

This PR updates the from_config method for model clients to ensure we
get the value from SecretStr.

Closes #5944