Add external programmers listing to board details commmand

[silvanocerza]

Please check if the PR fulfills these requirements

• 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)

• What kind of change does this PR introduce?
  A change to board details and to the gRPC interface to get a board details.

• What is the current behavior?
  The board details command doesn't like external programmers for a certain platform.

• What is the new behavior?
  Calling board details show also the list of external programmers supported by the board specified.
  Calling board details --list-programmers only shows the list of external programmers supported by the board.
  This information is also added to the gRPC interface.

• Does this PR introduce a breaking change?
  Nope.

• Other information:
  None.

---

See how to contribute

[silvanocerza]

Example output:

$ ./arduino-cli board details --fqbn arduino:avr:uno         
Board name:                Arduino Uno                                                 
FQBN:                      arduino:avr:uno                                             
Board version:             1.8.3                                                       

Official Arduino board:    ✔                                                           

Identification properties: VID:0x2341 PID:0x0043                                       
                           VID:0x2341 PID:0x0001                                       
                           VID:0x2A03 PID:0x0043                                       
                           VID:0x2341 PID:0x0243                                       

Package name:              arduino                                                     
Package maintainer:        Arduino                                                     
Package URL:               https://downloads.arduino.cc/packages/package_index.json    
Package website:           http://www.arduino.cc/                                      
Package online help:       http://www.arduino.cc/en/Reference/HomePage                 

Platform name:             Arduino AVR Boards                                          
Platform category:         Arduino                                                     
Platform architecture:     avr                                                         
Platform URL:              http://downloads.arduino.cc/cores/avr-1.8.3.tar.bz2         
Platform file name:        avr-1.8.3.tar.bz2                                           
Platform size (bytes):     4941548                                                     
Platform checksum:         SHA-256:de8a9b982477762d3d3e52fc2b682cdd8ff194dc3f1d46f4debdea6a01b33c14

Required tool:             arduino:avr-gcc                                                                             7.3.0-atmel3.6.1-arduino7

Required tool:             arduino:avrdude                                                                             6.3.0-arduino17          

Required tool:             arduino:arduinoOTA                                                                          1.3.0                    

Programmers:               Id                                                           Name                          
                           parallel                                                     Parallel Programmer           
                           usbasp                                                       USBasp                        
                           stk500                                                       Atmel STK500 development board
                           avrisp                                                       AVR ISP                       
                           arduinoisp                                                   ArduinoISP                    
                           avrispmkii                                                   AVRISP mkII                   
                           usbGemma                                                     Arduino Gemma                 
                           arduinoisporg                                                ArduinoISP.org                
                           arduinoasispatmega32u4                                       Arduino as ISP (ATmega32U4)   
                           usbtinyisp                                                   USBtinyISP                    
                           jtag3                                                        Atmel JTAGICE3 (JTAG mode)    
                           jtag3isp                                                     Atmel JTAGICE3 (ISP mode)     
                           atmel_ice                                                    Atmel-ICE (AVR)               
                           arduinoasisp                                                 Arduino as ISP                
                           buspirate                                                    BusPirate as ISP

$ ./arduino-cli board details --fqbn arduino:avr:uno --list-programmers         
Id                     Programmer name               
arduinoasispatmega32u4 Arduino as ISP (ATmega32U4)   
arduinoisporg          ArduinoISP.org                
stk500                 Atmel STK500 development board
buspirate              BusPirate as ISP              
arduinoisp             ArduinoISP                    
atmel_ice              Atmel-ICE (AVR)               
usbGemma               Arduino Gemma                 
jtag3isp               Atmel JTAGICE3 (ISP mode)     
usbtinyisp             USBtinyISP                    
usbasp                 USBasp                        
jtag3                  Atmel JTAGICE3 (JTAG mode)    
avrispmkii             AVRISP mkII                   
parallel               Parallel Programmer           
arduinoasisp           Arduino as ISP                
avrisp                 AVR ISP     ```