[common] add GetArrayLength() and GetArrayEnd() (#7413)

This commit adds two inline template functions to get the length of an
array and a pointer to the end of the array. These functions replace
similar macros that were used with OT core modules. The template
functions are safer to use since they would perform type checking at
compile time. The functions validate that the input variable is
indeed an array and unlike the macros they will not work with a
pointer variable.

tests/unit/test_child_table.cpp

@@ -31,6 +31,7 @@
 #include <openthread/config.h>
 
 #include "test_util.h"
+#include "common/array.hpp"
 #include "common/code_utils.hpp"
 #include "common/instance.hpp"
 #include "thread/child_table.hpp"
@@ -290,7 +291,7 @@ void TestChildTable(void)
         },
     };
 
-    const uint16_t testListLength = OT_ARRAY_LENGTH(testChildList);
+    const uint16_t testListLength = GetArrayLength(testChildList);
 
     uint16_t testNumAllowedChildren = 2;