Address ENG-10683.
This mod was messy - HTTP handlers/responders were mixed with helper functions with misleading names.
Also, it was getting tough with references lifetimes, which could make trouble in next diffs with copying logs.
- Moved Blob/Holder-specific functions to a submodule. Renamed them to be more accurate
- Moved Multipart-processing utilities to a trait with impl for Multipart object. This made syntax cleaner.
Depends on D14681