mavlink: add 'mavlink status streams' command

Displays all enabled streams and their configured rates.
Example output:
instance #1:
        Name                Rate Config (current) [Hz] Message Size (if active) [B]
        HEARTBEAT                       1.00 (1.000)    21
        STATUSTEXT                     20.00 (6.320)
        COMMAND_LONG                  unlimited
        ALTITUDE                        1.00 (0.316)
        ATTITUDE                       25.00 (7.900)    40
        ATTITUDE_TARGET                10.00 (3.160)    49
        ESTIMATOR_STATUS                1.00 (0.316)    44
        EXTENDED_SYS_STATE              1.00 (0.316)    14
        GLOBAL_POSITION_INT            10.00 (3.160)    40
        GPS_RAW_INT                     1.00 (0.316)    62
        HOME_POSITION                   0.50 (0.158)
        RC_CHANNELS                     5.00 (1.580)
        SERVO_OUTPUT_RAW_0              1.00 (0.316)    49
        SYS_STATUS                      5.00 (1.580)    43
        SYSTEM_TIME                     1.00 (0.316)    24
        VFR_HUD                        25.00 (7.900)    32
        WIND_COV                        2.00 (0.632)