Handle PINGs in QUIC connection. Currently, sending and retransmission of PINGs are handled by control frame manager (and session). Protected by FLAGS_quic_reloadable_flag_quic_let_connection_handle_pings. PiperOrigin-RevId: 335072367 Change-Id: I79f20404656beee3d9404c1ccaa8ef2f00af567c