Added 'source override' feature to compile command.

[cmaglie]

This feature allows to selectively "override" the content of a sketch. Overridden files will not be read from disk as usual but fetched directly from gRPC parameters (if the CLI is running as a daemon) or from a .json file containing the new source code (if the CLI us running from the command line).

This feature is useful for IDE that may have unsaved changes: in this case, the "dirty" content of the text editor may be verified/compiled anyway without the need to save the sketch on disk.

The same feature from the command line is achieved by storing the overridden files in a temporary json file and by using the --source-override path/to/json option for compile.

here an example transcript:

:~/Arduino/TestTemps$ ls -l
totale 12
-rw-r--r-- 1 cmaglie cmaglie 70 dic  3 17:25 overrides.json
-rw-r--r-- 1 cmaglie cmaglie 13 dic  3 17:23 Second.ino
-rw-r--r-- 1 cmaglie cmaglie 40 dic  3 17:23 TestTemps.ino

:~/Arduino/TestTemps$ cat TestTemps.ino 
void setup() {
}
void loop() {
  a();
}

:~/Arduino/TestTemps$ cat Second.ino 
void b() { }

:~/Arduino/TestTemps$ arduino-cli compile -b arduino:avr:uno 
/home/cmaglie/Arduino/TestTemps/TestTemps.ino: In function 'void loop()':
/home/cmaglie/Arduino/TestTemps/TestTemps.ino:4:3: error: 'a' was not declared in this scope
   a();
   ^

Error during build: exit status 1

:~/Arduino/TestTemps$ cat overrides.json
{ "overrides": {
  "Second.ino": "void b() { }\nvoid a() { }\n"
  }
}

:~/Arduino/TestTemps$ arduino-cli compile -b arduino:avr:uno --source-override overrides.json 
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.

:~/Arduino/TestTemps$ arduino-cli compile -b arduino:avr:uno --source-override overrides.json --format json
{
  "compiler_out": "Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.\nGlobal variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.\n",
  "compiler_err": "",
  "builder_result": {
    "build_path": "/tmp/arduino-sketch-A54D0E2639DDA6E3481AB05BD0D564A6",
    "executable_sections_size": [
      {
        "name": "text",
        "size": 444,
        "maxSize": 32256
      },
      {
        "name": "data",
        "size": 9,
        "maxSize": 2048
      }
    ]
  },
  "success": true
}

:~/Arduino/TestTemps$ cat /tmp/arduino-sketch-A54D0E2639DDA6E3481AB05BD0D564A6/sketch/TestTemps.ino.cpp
#include <Arduino.h>
#line 1 "/home/cmaglie/Arduino/TestTemps/TestTemps.ino"
#line 1 "/home/cmaglie/Arduino/TestTemps/TestTemps.ino"
void setup();
#line 3 "/home/cmaglie/Arduino/TestTemps/TestTemps.ino"
void loop();
#line 1 "/home/cmaglie/Arduino/TestTemps/Second.ino"
void b();
#line 2 "/home/cmaglie/Arduino/TestTemps/Second.ino"
void a();
#line 1 "/home/cmaglie/Arduino/TestTemps/TestTemps.ino"
void setup() {
}
void loop() {
  a();
}

#line 1 "/home/cmaglie/Arduino/TestTemps/Second.ino"
void b() { }
void a() { }

[matthijskooijman]

I looked over the code, looks good to me. Haven't looked in much detail or done testing, though.