Allow placeholders in the authorization context of a connection

[dguggemos]

Currently the subjects of an authorization context are defined as a fixed string, which means you have to grant this subject access to all devices of a connection. To provide more flexibility we want to introduce placeholders in the authorization subject which are replaced before the signal is processed in Ditto.
For example you can define the authorization context of a connection as:

...
"authorizationContext": ["ditto:{{ header:device-id }}"]
...

The placeholder {{ header:device-id }} is then replaced by the value of the device-id header . If a placeholder cannot be resolved, e.g. because the specified header is missing, the message is rejected.