[Generator] Async bodies + swift-http-types adoption

[czechboy0]

Motivation

Generator changes of the approved proposals #255 and #254.

Modifications

• Adapts to the runtime changes.
• Most changes are tests updating to the new generated structure.
• As usual, easiest to start with the diff to the file-based reference tests to understand the individual changes, and then review the rest of the PR.
• To see how to use the generated code, check out some streaming examples in https://github.com/apple/swift-openapi-generator/pull/245/files#diff-2be042f4d1d5896dc213e3a5e451b168bd1f0143e76753f4a5be466a455255eb

Result

Generator works with the 0.3.0 runtime API of.

Test Plan

Adapted tests.