Add body field to builders that accept a body

[russcam]

For APIs that accept a request body, the builder struct should expose a function for setting a body for the request. The input parameter should probably be a type that implements serde's Serialize trait, to allow a user to supply their own type for a request, or use serde's json! macro and serde_json::Value.

[russcam]

Working on this now

[russcam]

Implemented in 32e1e0401406882a9feb3daffc79ab3b5dbf8046