[Feature] Add Support for Item Previews

[Kinglykrab]

Description

• Adds support for the Item Preview window in merchants.
• Credit to @dannuic.

Type of change

• New feature

Testing

Checklist

• I have tested my changes
• I have performed a self-review of my code. Ensuring variables, functions and methods are named in a human-readable way, comments are added only where naming of variables, functions and methods can't give enough context.
• I own the changes of my code and take responsibility for the potential issues that occur

[neckkola]

Good morning. Kingly, I wonder if we should move the struct verification into the decode/encoders so that when we ever get the other clients online, we can have source be generic, and do the packet differences there? It looks like we are doing most of that, at least on the decode side within client_packet. I also see RoF changes though no RoF opcode. Do we need to add to patch_rof?