config service: joins and provide

[adecaro]

This issue is about introducing the following two features:

• A function to join keys in a safe way to form keys like key1.key2.key3 like we do when joining file path components.
• A function to load a raw presentation of a yaml tree before merging into the main configuration.

This is propaedeutic to the ability of updating the configuration at run-time.

These features will be used also by the token-sdk: hyperledger-labs/fabric-token-sdk#1209