update_function_alias additional arguments

[jonthegeek]

The UpdateAlias endpoint accept arguments that we don't deal with:

• RevisionId (only update if the alias's revision ID matches this; an additional safety that name + alias is pointing to the right thing)
• RoutingConfig (more info in its own doc)

Adding RevisionId should be trivial, but adding RoutingConfig will evidently be more complicated. Consider doing them one at a time.