make DefaultStorageKeyAllocator::Format protected so the class can be…

… extended (#20326)

If integrating the matter PersistentStorageDelegate into an out-of-tree
implementation, it is useful to extend DefaultStorageKeyAllocator to
make it clear to maintainers that there is a single, shared key space
and that keys must avoid collision with those used in the sdk.

However, extending the PersistentStorageDelegate class is awkward
because the Format method is private.

Make it protected to streamline extending the class.

src/lib/support/DefaultStorageKeyAllocator.h

@@ -114,7 +114,7 @@ class DefaultStorageKeyAllocator
     // Event number counter.
     const char * IMEventNumber() { return Format("g/im/ec"); }
 
-private:
+protected:
     // The ENFORCE_FORMAT args are "off by one" because this is a class method,
     // with an implicit "this" as first arg.
     const char * ENFORCE_FORMAT(2, 3) Format(const char * format, ...)
@@ -126,6 +126,7 @@ class DefaultStorageKeyAllocator
         return mKeyName;
     }
 
+private:
     char mKeyName[PersistentStorageDelegate::kKeyLengthMax + 1] = { 0 };
 };