debug: Slightly refactored custom_configs section of GetDebugConfigResponse

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

This PR changes the JSON produced by debug --info --format json command, making it more clear the section about custom configurations.
Previously the cortex-debug configuration was under the "cortex-debug_custom_configuration" field, now there is a section "custom_configs" with a "cortex-debug" subfield.
This change also allows for a future expansion of the number of plugins supported.

What is the current behavior?

$ arduino-cli debug -b arduino:samd:mkr1000 --info --format json -P edbg
{
  "executable": "/tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf",
  "toolchain": "gcc",
  "toolchain_path": "/home/megabug/.arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4/bin/",
  "toolchain_prefix": "arm-none-eabi-",
  "server": "openocd",
  "server_path": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd",
  "server_configuration": {
    "path": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd",
    "scripts_dir": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/share/openocd/scripts/",
    "scripts": [
      "/home/megabug/.arduino15/packages/arduino/hardware/samd/1.8.13/variants/mkr1000/openocd_scripts/arduino_zero.cfg"
    ]
  },
  "cortex-debug_custom_configuration": {
    <--- Corted-debug configuration was here
  },
  "programmer": "edbg"
}

What is the new behavior?

$ arduino-cli debug -b arduino:samd:mkr1000 --info --format json -P edbg
{
  "executable": "/tmp/arduino/sketches/002050EAA7EFB9A4FC451CDFBC0FA2D3/Blink.ino.elf",
  "toolchain": "gcc",
  "toolchain_path": "/home/megabug/.arduino15/packages/arduino/tools/arm-none-eabi-gcc/7-2017q4/bin/",
  "toolchain_prefix": "arm-none-eabi-",
  "server": "openocd",
  "server_path": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd",
  "server_configuration": {
    "path": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/bin/openocd",
    "scripts_dir": "/home/megabug/.arduino15/packages/arduino/tools/openocd/0.10.0-arduino7/share/openocd/scripts/",
    "scripts": [
      "/home/megabug/.arduino15/packages/arduino/hardware/samd/1.8.13/variants/mkr1000/openocd_scripts/arduino_zero.cfg"
    ]
  },
  "custom_configs": {
    "cortex-debug": {
      <--- Corted-debug configuration now is here
    }
  },
  "programmer": "edbg"
}

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (64f1853) 64.56% compared to head (cbbb15a) 64.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2396      +/-   ##
==========================================
+ Coverage   64.56%   64.58%   +0.01%     
==========================================
  Files         207      207              
  Lines       19632    19634       +2     
==========================================
+ Hits        12676    12681       +5     
+ Misses       5865     5863       -2     
+ Partials     1091     1090       -1     

Flag	Coverage Δ
unit	64.58% <85.71%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
commands/debug/debug_info.go	73.42% <100.00%> (ø)
internal/cli/debug/debug.go	76.29% <77.27%> (+2.61%)	⬆️

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