Add support for `RuntimeStruct`'s initialized from a protocol factory's protocol singleton

[vkottler]

the last step for getting Protocol instances wired to UI.

add support for a config option for any RuntimeStruct so that it can find a protocol factory (from configured import string), get its singleton instance and set up the channel environment's channels + fields + the struct's byte order and .array instance

[vkottler]

running into a scenario described on libre-embedded/ifgen#29 while testing so may implement that first