Use QpackEncoder::InstructionWithValues to refactor QpackInstructionEncoder interface. This change is inspired by rch's suggestion at cr/258561443. gfe-relnote: n/a, change to QUIC v99-only code. Protected by existing disabled gfe2_reloadable_flag_quic_enable_version_99. PiperOrigin-RevId: 277699504 Change-Id: Iebde22504ae29f2307433458defb9f308bc5cb64