Kafka sink raises error when `key_path` is specified in the config

[StewartJingga]

Describe the bug Kafka sink fails to add key to its message due to changes in Metadata models. (Urn is now nested inside Resource)

To Reproduce Steps to reproduce the behavior:

1. Create a recipe with kafka sink
2. Set key_path value
3. Run recipe
4. See error

Expected behavior The recipe should be working properly with key_path and key should be published

Desktop (please complete the following information):

• OS: [e.g. macOS]
• Version [e.g. 10.15]