Replace "Either" trait type with "Union"

[rahulporuri]

This PR replaces the Either trait with the Union trait.

A new Union trait type has been added. This is intended as a simpler replacement for the existing Either trait type, which will eventually be deprecated.

See https://github.com/enthought/traits/releases/tag/6.1.0.

See similar PR in traitsui enthought/traitsui#1581

[aaronayres35]

LGTM, but same comments as from my review on enthought/traitsui#1581

I believe these traits used to default to None but now they default to "". This possibly is no problem at all, but we should be aware of the change.

[mdickinson]

I think we can reasonably assume that the protocol and factory traits are always provided when creating a ServiceOffer, so that we don't have to worry too much about default values.

To back that up, here's where the protocol is used, and where we dispatch based on whether it's a string or not; it's clear that if the default value of None reached that code, it would do the wrong thing:

envisage/envisage/service_registry.py

Lines 225 to 232 in d62af08

	if isinstance(protocol_or_name, str):
	name = protocol_or_name
	else:
	name = "%s.%s" % (
	protocol_or_name.__module__,
	protocol_or_name.__name__,
	)

And for the factory, here's where the string versus callable is resolved:

envisage/envisage/service_registry.py

Lines 262 to 265 in d62af08

	if isinstance(obj, str):
	obj = ImportManager().import_symbol(obj)
	obj = obj(**properties)

IOW, LGTM.

[rahulporuri]

thanks for the review @mdickinson and @aaronayres35