POC Dynamic shell completion for fqbn flag

[umbynos]

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• Tests for the changes have been added (for bug fixes / features)
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• What kind of change does this PR introduce?

Dynamic shell completion

• What is the current behavior?

Now the shell completion is only static and only works by completing sub-commands and flags hard-coded in the CLI code.

• What is the new behavior?

With cobra 1.2.0 they added dynamic shell completion for nouns and for flags (see here and here).
The goal of this Proof Of Concept PR is to implement the dynamic completion for the arduino-cli compile --fqbn flag and sub-command, by retrieving the list of the fqbn (similar to board listall command).

umberto@XPS-15:~/Lavoro/arduino-cli$ ./arduino-cli compile --fqbn <TAB><TAB>
mbed_nano:nano33ble                 samd:arduino_zero_native            samd:mkrwan1300                     samd:mzero_pro_bl_dbg
mbed_nano:nanorp2040connect         samd:mkr1000                        samd:mkrwan1310                     samd:nano_33_iot
megaavr:nona4809                    samd:mkrfox1200                     samd:mkrwifi1010                    samd:tian
megaavr:uno2018                     samd:mkrgsm1400                     samd:mkrzero                        
samd:adafruit_circuitplayground_m0  samd:mkrnb1500                      samd:mzero_bl                       
samd:arduino_zero_edbg              samd:mkrvidor4000                   samd:mzero_pro_bl    

In order to make this happen, I bumped the cobra dependency to the latest version (1.2.1) in #1430
I used the newer GenBashCompletionV2 command to generate the completion for bash (to support dynamic completion)

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

• Other information:

---

See how to contribute

[per1234]

This flag is not required due to the ability to define it in the sketch metadata via arduino-cli board attach:

$ ./arduino-cli version
arduino-cli.exe alpha Version: 0.19.0 Commit: 56419ecd Date: 2021-09-02T14:47:35Z

$ ./arduino-cli sketch new /tmp/FooSketch
Sketch created in: C:\Users\per\AppData\Local\Temp\FooSketch

$ ./arduino-cli board attach arduino:avr:uno /tmp/FooSketch
Selected fqbn: arduino:avr:uno

$ ./arduino-cli compile /tmp/FooSketch
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.

[umbynos]

you are right, I thought about it later... I just wanted to experiment with MarkFlagRequired function. We can use this somewhere else:
basically if you type completion <tab><tab> the completion suggest at first only the required flags, so the user is prompted to use them.

[umbynos]

BTW I reverted it 😄