Implement expand_kwargs() against a literal list

[uranusjr]

This adds literal support to expand_kwargs():

task.expand_kwargs([{"arg": 1}, {"arg": 2}, ...])

# Literal list can contain either literal dict or XComArg.
task.expand_kwargs([xcom_arg, {"arg": 2}, ...])

This matches the expand() interface.

This needs #25924 to go in first and is submitted for CI now.

[ashb]

(Attempted to resolve conflicts vis GH, may not have worked out very well)