slapos.recipe.request: Add requestoptional.serialised
It is combination of requestoptional and serialised.
Tasks:
-
make Serialised a mix-in class (see https://lab.nexedi.com/nexedi/slapos/merge_requests/381#note_65076 ) -
bootstrap tests (see https://lab.nexedi.com/nexedi/slapos/merge_requests/381#note_65100 )