SDK support for config map

[daviddrysdale]

#1004 introduced a mechanism where a collection of <name, file-contents> pairs are injected into the initial Node of an application, to allow runtime config info to be acquired by the Application.

It would be nice to have wider support to make this easier to re-use, including:

• Helper code in the SDK to read the config map from the Node's initial handle and reconstruct into Rust types (followed by a channel_close).
• A variant of the SDK's entrypoint! macro to do this automatically for Nodes that need config info.
• ABI documentation of the expected behaviour and format of the messages arriving on the initial Node's initial channel.
• SDK documentation explaining how to use the feature (probably in docs/programming-oak.md).