Encoding/decoding async sequences for JSON Lines, JSON Sequence, and Server-sent Events

[czechboy0]

It'd be good to have encoding and decoding async sequences for JSON Lines, JSON Sequence, and Server-sent Events in the runtime library.

The generator wouldn't change, adopters just could chain these sequences onto an HTTPBody they got from a streaming API and do easy event streams, pub/sub, and so on.

[czechboy0]

Should also include examples of how to use these in our Examples directory, both client and server for all 3.

[czechboy0]

Code works here: apple/swift-openapi-runtime#91

Will clean it up, document, and write up a proposal next week, to get feedback on the API.

[czechboy0]

Landed examples in #494.