Load key contents from suitable source at runtime

[paulhowardarm]

The key-broker server currently initialises with a single hard-coded key whose name/ID is skywalker and whose value decodes as ASCII to the string May the force be with you.

This is used in simple demonstrators of the key broker, and is fine for such straightforward interactions.

This feature request is to replace the hard-coded mechanism with a more general way of importing keys into the server at start-up time. This would allow the broker to support multiple keys, and for the admin to specify their contents. This could be as simple as a JSON or TOML file on disk.

An example input file must be supplied, and this example file must establish the same skywalker key that is currently hard-coded. This is so that there is no change in the default out-of-box behaviour. This is important because existing client demos will expect this key to exist.