createSourceEventStream: introduce named arguments and deprecate positional arguments (#3645)
BACKPORT OF #3634
Deprecates the positional arguments to createSourceEventStream, to be removed in the next major version, in favor of named arguments.
Motivation:
1. aligns createSourceEventStream with the other exported entrypoints graphql, execute, and subscribe
2. allows simplification of mapSourceToResponse
suggested by @IvanGoncharov