EmberAfAttributeMetadata size field is always 0 for external attributes

[mrjerryjohns]

Problem

When implementing emberAfExternalAttributeReadCallback, the maxReadLength field is always 0. This is not the case for non-external attributes.

This makes it difficult for applications to really know how much space is available, especially for variable-length types like octet-strings and char strings.

This is caused by use of that field as an offset as well into the store, where the offsets are summed up in the attribute metadata table. This has to be 0 in that situation since no space is actually allocated.

[bzbarsky-apple]

So I just checked, and in emAfReadOrWriteAttribute when computing the offsets we have:

                            if (!(am->mask & ATTRIBUTE_MASK_EXTERNAL_STORAGE) && !(am->mask & ATTRIBUTE_MASK_SINGLETON))
                            {
                                attributeOffsetIndex = static_cast<uint16_t>(attributeOffsetIndex + emberAfAttributeSize(am));
                            }

so we could have nonzero sizes here and not mess up the indexing.

What we would need to make sure of is that the cluster/endpoint sizes are not affected. Let me see what I can do here...

[bzbarsky-apple]

@mrjerryjohns I think project-chip/zap#558 should do what we want here.