client: Add optional custom User-Agent to HTTPRemoteStore
client: Add optional custom User-Agent to HTTPRemoteStore

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>
cyli/gotuf,endophage/gotuf,dmcgowan/gotuf,runcom/gotuf,theupdateframework/go-tuf,diogomonica/gotuf