Add more TLV nw-comm structs

[ivmarkov]

The PR does two three things:

• Defines most of the structs necessary for all the supported NW commissioning attributes and commands to actually be processed; this might be superseded in future by the IDL-to-Rust generator, but until it is fully functional, we'll need these
• Exposes three different cluster metadata structs: ETH_CLUSTER (formally CLUSTER), THR_CLUSTER and WIFI_CLUSTER (as the set of commands an attributes differ depending on what is the used operational network)
• Renames the existing NwCommCluster handler to EthNwCommCluster, as it is really Ethernet specific. Users (like me) who would like to use other networks, like Wifi (or Thread) need a different handler provided by themselves.

[ivmarkov]

(There will be a follow-up PR that generifies the Root endpoint handler and metadata, depending on the type of operational network the user is planning to utilize.)