Callback to resolve a URIVariable by a given variable name.
The variable name is always resolved to a simple string and the returned
value should not be encoded or otherwise processed for the URI.
To conform to RFC 6570 the callback MUST always produce the same variables
for any given variable name. The values MUST be determined before template
expansion.
Callback to resolve a URIVariable by a given variable name. The variable name is always resolved to a simple string and the returned value should not be encoded or otherwise processed for the URI. To conform to RFC 6570 the callback MUST always produce the same variables for any given variable name. The values MUST be determined before template expansion.