Unify labelName and authorUsername graphql arguments
Related to #382969 (comment 1290963871)
There is a difference in labelName/authorUsername param names for issues/epics resolvers, I think current status is:
default listing | negated | unioned
issues labelName (array) | labelName (array) | labelNames (array)
epics labelName (array) | labelName (array) | labelName (array)
issues authorUsername (single) | authorUsername (single) | authorUsernames (array)
epics authorUsername (single) | authorUsername (single) | authorUsername (array)
Recently I added singular unioned
arguments for epics (singular to be consistent with existing epic params), but this adds difference for unioned
filter compared to issues. Ideally we should unify these to be same.