feat: add custom parameter based on a map of values

[daantimmer]

Currently the PARAMETER macro only accepts a regex string. But there are quite some occasions where a custom parameter is nothing more than mapping a string to values. This can for example be used to create a custom parameter based on enums:

// somewhere
enum class State{
 connected,
 disconnected,
 standby,
 shutdown
}

namespace{
  const std::map<std::string, State> stateMap{
   {"connected", State::connected},
   {"disconnected", State::disconnected},
   {"standby", State::standby},
   {"shutdown", State::shutdown},
  };
}

PARAMETER_MAP("state", stateMap, true);

// PARAMETER_MAP does the heavy lifting of transforming all the keys from the provided
// map in to a regex that would look like: `(connected|disconnected|standby|shutdown)`
// it also takes care of creating a simple implementation that takes a string
// and uses the .at function on the provided map using the captured argument
// it could potentially look something like this (pseudo-code-ish)
#define PARAMETER_MAP(name, map, snippet) \
PARAMETER(decltype(map)::value_type, (name, transformKeysToRegex(map), snippet), (const std::string& key)) \
{ \
  return map.at(key); \
}