Implemented pcb_gerbers_layers parameter for Gerber files generation

[vikulin]

This parameter has an empty default value. All layers are generated when no value assigned.

Example of usage:

      - name: Run KiCad export
        uses: vikulin/kicad-action@main
        with:
          kicad_sch: gamma-spectrometer.kicad_sch
          sch_pdf: true # Generate PDF
          sch_pdf_file: docs/open-gamma-kit-${{ github.ref_name }}.pdf
          sch_bom: false
          kicad_pcb: gamma-spectrometer.kicad_pcb
          pcb_gerbers: true # Generate Gerbers
          pcb_gerbers_file: build/open-gamma-kit-gerber-files-${{ github.ref_name }}.zip
          pcb_gerbers_layers: F.Cu,B.Cu,F.SilkS,B.SilkS,F.Mask,B.Mask,Edge.Cuts