[Multipart] Add public types

[czechboy0]

Motivation

Add the public types approved in SOAR-0009.

Modifications

Added the public types and make other changes outlined in SOAR-0009.

Result

Most of the required runtime types are now in place.

Test Plan

Added unit tests for the new code, where it made sense.

⚠️ The pipeline pull request validation (api breakage) is failing with:

1 breaking change detected in OpenAPIRuntime:
  💔 API breakage: constructor Configuration.init(dateTranscoder:) has removed default argument from parameter 0
** ERROR: ❌ Breaking API changes detected.

but that seems to be a false positive in the tool, as there is now a newer initializer that you can use as .init(), .init(dateTranscoder:), .init(multipartBoundaryGenerator:), or init(dateTranscoder:multipartBoundaryGenerator:), so no existing code could be broken by this change.

[simonjbeaumont]

Minor changes, otherwise LGTM.

[simonjbeaumont]

LGTM, feel free to land once you appease the CI (probably my bad suggestions causing formatting complaints; didn't check). I guess you'll need to merge over the API checker, which you said is a false positive.