treewide: add copyright/license notices

pkg/connector/backfill.go

pkg/connector/ids.go

pkg/connector/linkedinfmt/attributedtext_parse_test.go

pkg/connector/matrix.go

pkg/connector/sync.go

pkg/connector/tomatrix.go

pkg/linkedingo/attributedtext.go

pkg/linkedingo/conversations.go

pkg/linkedingo/media.go

pkg/linkedingo/messages.go

pkg/linkedingo/reactions.go

pkg/linkedingo/realtime.go

pkg/linkedingo/receipts.go

pkg/linkedingo/request.go

pkg/linkedingo/typing.go

pkg/linkedingo/urn.go