[IM] Use DataModel::Encode in EncodeScalarAttributeWritePayload for complex types

[erjiaqing]

Problem

Need to support writing complex types.

Change overview

• Replace payload write logic by DataModel::Encode

Testing

• New unit tests added to verify WriteClient can handle writing complex types.

[woody-apple]

@saurabhst @jelderton @jepenven-silabs @jmartinez-silabs ?

[bzbarsky-apple]

This is at least not wrong now, though I still think the compare should be against the actual data we meant to send (i.e. the string) not an intermediate representation (the CharSpan), if it's possible....

By the way it might be worth using CharSpan instead of Span in general....