Develop workaround in endpoint_config.zapt for "attribute accessor" storage class

[jonsmirl]

Zap does not currently support defining storage classes for attributes. While this is on the road map to get fixed in Zap, a temporary solution is needed.

The temporary solution is to add dedicated code in endpoint_config.zapt which will detect the Matter attributes implemented via the "attribute accessor" storage class and generate the correct code. (A way to flag this would be to look for type='ARRAY" and no length specified).

Later when Zap catches up this code can be removed.

[bzbarsky-apple]

Note to self: Need to do this for things with array and struct types.

[bzbarsky-apple]

Update: This is done for lists in #13846.

For structs, code generation will fail at the moment for non-external structs, though the error will be a little inscrutable. A better fix is blocked on project-chip/zap#391

Furthermore, we have a way of flagging attributes as "must have external storage" in the SDK, in src/app/zap-templates/zcl/zcl.json.

So not sure how much is left to do here immediately.