[neighbor-table] add 'NeighborTable' class (#5354)

This commit adds a `NeighborTable` class which provides helper methods
dealing with neighbor, e.g., finding a `Neighbor` (i.e., a `Child`
from `ChildTable`, a `Router` from `RouterTable`, or parent or parent
candidate entries), or iterating through all neighbors.  These methods
use the newly added `Neighbor::AddressMacther` to simplify the
implementation. The `NeighborTable` methods replace the similar ones
in `Mle` and `MleRouter`.

src/core/api/thread_api.cpp

@@ -304,7 +304,7 @@ otError otThreadGetNextNeighborInfo(otInstance *aInstance, otNeighborInfoIterato
 
     OT_ASSERT((aInfo != nullptr) && (aIterator != nullptr));
 
-    return instance.Get<Mle::MleRouter>().GetNextNeighborInfo(*aIterator, *static_cast<Neighbor::Info *>(aInfo));
+    return instance.Get<NeighborTable>().GetNextNeighborInfo(*aIterator, *static_cast<Neighbor::Info *>(aInfo));
 }
 
 otDeviceRole otThreadGetDeviceRole(otInstance *aInstance)