Server function streaming with serializable types

[ealmloff]

This PR handles serialization and deserialization for custom types with streaming server functions (#1284)

It adds a new StreamingJson encoding which serializes each item in the stream with serde_json. The implementation is a fairly straight forward combination of the streaming bytes and json codecs with two exceptions:

1. The content type is the default streaming type instead of JSON because if the chunks are read as merged chunks they cannot be parsed as valid json
2. When using a custom stream type with StreamingJson, this PR requires the bound Stream<Item=T> in order to make the T type used in the trait implementation. We don't use the bound, but it makes the T type constrained. Alternatively, I could add an unused generic to StreamingJson or FromReq, but I would imagine both of those would effect user code more

[gbj]

This looks good to me. Thanks very much for the addition!