Add constructor for ConditionAccessor that does not create a property

As @graven proposed in Moore!1301 (comment 5248825), it is useful to be able to create ConditionAccessors without associating them to a property (it's actually uncommon that we want to change a condition key at configuration time).

This MR adds a constructor for ConditionAccessor (both DetDesc and DD4hep variants) that does not imply a property. The two options will be:

struct MyAlg: ... {
  // condition with property
  ConditionAccessor m_cond1{this, "PropName", "Condition/Path/...", "my configurable condition"};
  // condition without property
  ConditionAccessor m_cond2{this, "Condition/Path/..."};
};

To-do:

• modify the examples in FunctionalConditionAccessor.cpp to use the new constructor