"kn service export" should also export service annotations

[rhuss]

Bug report

At the moment, kn export does not export annotations.

Expected behavior

User provided annotations should be exported as well (like with kn service create --annotation). Maybe filter out some automatically added annotations (if known), but that might be difficult.

Adding an E2E test is a plus.

Steps to reproduce the problem

• Create a service with annotation: kn service create mysvc --annotation myanno=value someimage
• Export its: kn service export mysvc
• Check whether the annotation is included.

[itsmurugappan]

/assign @itsmurugappan

[itsmurugappan]

@rhuss this was fixed by #819.