Add VOA range and actual VOA for optical-amplifier.yang and wavelength-router.yang

[HubertYuanjing]

A typical amplifier will have an integrated Variable Optical Attenuator integrated. So I suggest to add voa target value and actual voa attenuation into optical-amplifier.yang config and state container, details are:

config & state container:
    leaf target-voa {
      type decimal64 {
        fraction-digits 2;
        range 0..max;
      }
      units dB;
      description
        "Voa attenuation control";
    }

state container only:
    container actual-voa {
      description
        "The actual attenuation applied by the voa integrated with in amplifier in units of 0.01dB. If avg/min/max statistics are not supported, just supply the instant value";

      uses oc-types:avg-min-max-instant-stats-precision2-dB;
    }

Also, for Wavelength router application, attenuation for media channel should also be monitored and configurable, so I propose to add target voa and actual voa attenuation into wavelength-router.yang config and state container, details are:

config & state container:
    leaf target-voa {
      type decimal64 {
        fraction-digits 2;
        range 0..max;
      }
      units dB;
      description
        "Voa attenuation control";
    }

state container only:
   container actual-voa {
      description
        "The actual attenuation applied by the voa for specified media channel in units of 0.01dB. If avg/min/max statistics are not supported, just supply the instant value";
      uses oc-types:avg-min-max-instant-stats-precision2-dB;
    }

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