Add Online Help link to gRPC interface's Platform

[MatteoPologruto]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• 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)

What kind of change does this PR introduce?

Code enhancement

What is the current behavior?

The Online Help link present in the package-index.json cannot be retrieved.

What is the new behavior?

It is possible to have access to Online Help using core search --format json.

PS C:\Users\m.pologruto\Desktop\Arduino\arduino-cli> .\arduino-cli core search avr --format json
[
...
{
   "id": "littleBits:avr",
   "latest": "1.0.0",
   "name": "littleBits Arduino AVR Modules",
   "maintainer": "littleBits Electronics",
   "website": "http://www.littlebits.cc/bits/w6-arduino",
   "email": "support@littlebits.cc",
   "boards": [
     {
       "name": "littleBits w6 Arduino module"
     }
   ],
   "type": [
     "Arduino@Heart"
   ],
   "help": {
     "online": "http://www.littlebits.cc/arduino-help"
   }
 }
]

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

No

[per1234]

Thanks Matteo!

[umbynos]

Code LGTM