Add generic function for updating the annotations only

[Liujingfang1]

Add a function to update the annotation of an object. Here is how it should be used.

obj := client.Get()

var newAnnotations map[string]string
// get the new annotations that obj should have
// not including the last-applied-config annotation

UpdateAnnotation(obj, newAnnotations)

client.Update(obj)

Also add unit tests for the new function.

[Liujingfang1]

/assign @seans3

[seans3]

Small nitpicks.

[seans3]

/lgtm