Add support for json, adoc and md sketch files extensions

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• 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?

Enhances gRPC interface.

• What is the current behavior?

LoadSketch gRPC interface function doesn't return Sketch files in the root folder that have .json, .adoc or .md extension.

• What is the new behavior?

LoadSketch gRPC interface function now returns Sketch files in the root folder that have .json, .adoc or .md extension.
This only affects the gRPC interface.

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

Nope.

• Other information:

None.

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[matthijskooijman]

The description for this PR says:

LoadSketch gRPC interface function now returns Sketch files in the root folder that have .json, .adoc or .md extension.

However, AFAICS this also returns such files in subdirectories. Is this intentional?

Also, what is the intended usecase here? To allow the user to edit/view adoc or md files when editing/viewing a sketch? How about json files? Is this intended for json data files in the sketch? Or for sketch.json? If the latter, should that maybe just that file be included and not all .json files (asking in relation to #1201, about breakage when a compilation_commands.json is present)?

This only affects the gRPC interface.

Is this really true? AFAICS it also causes these files to be copied into the build directory (or maybe that's a side effect of #1353?), which could influence the build, and can certainly influence performance.

[per1234]

Or for sketch.json?

It is handy for the debug_custom.json file used to add support to the Arduino IDE 2.x debugger for non-CMSIS DAP compliant debuggers. The file must be added to the sketch.