[Multipart] Add a raw parts to frames serializer sequence.

[czechboy0]

Motivation

Depends on #74.

The opposite direction of #74, goes from raw multipart parts to multipart frames.

Modifications

Same dance, a state machine, a serializer, and an async sequence.

Result

We can convert from raw multipart parts to multipart frames.

Test Plan

Added unit tests for all 3 layers.