WMS GetCapabilities could announce offered VARIABLES

[lubojr]

Custom parameter VARIABLES can already be used to set user-provided variables into browse rendering expression. However these options need to be specially documented for each browse type (there they are set as var(interval, 30) for example).

EOxServer WMS GetCapabilities response could be extended to also list the possible variables from each browse expressions.

Implementation-wise, this would mean parsing each individual browse type expression, extracting the contained variables and adding them to the WMS XML response under a new custom key (for each layer).

Optionally, in order to offer even more context to the user - like variable description, allowed or meaningful input values etc., it would be helpful if this concept of offered variables could be extended to allow adding extra information during browse type creation by the operator to be shown to the user in GetCapabilities for each variable.