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- Using configmap allows better handling of config types
- The CLI "instances" now caches all the configuration at Create-time
- Setting are now only accessible via gRPC calls
- Helper methods have been implemented to access all key/value pairs

Now the configuration is kept inside the arduinoCoreServiceImpl struct.

No more direct access to the configuration, the required config values
are passed as arguments or available trough struct fields.

Viper object is now embedded into a new configuration.Setting object.
This would allow to make better getters and setters methods in the next
commits.

HTTP downloader configuration is generated using the configuration.

ARDUINO_DATA_DIR, ARDUINO_DOWNLOADS_DIR, and ARDUINO_SKETCHBOOK_DIR are
old variables kept just for backward compatibility.

Inlining methods in ArduinoCoreServiceImpl (part 16: CleanDownloadCacheDirectory, CheckForArduinoCLIUpdates)

Inlining methods in ArduinoCoreServiceImpl (part 15: SupportedUserFields, ListProgrammersAvailableForUpload, EnumerateMonitorPortSettings)

Inlining methods in ArduinoCoreServiceImpl (part 13: NewSketch, LoadSketch, SetSketchDefaults, ArchiveSketch)

Inlining methods in ArduinoCoreServiceImpl (part 11: LibraryDownload, LibraryInstall, LibraryUpgrade, LibraryUninstall, LibraryUpgradeAll, LibraryResolveDependencies, LibrarySearch, LibraryList, ZipLibraryInstall, GitLibraryInstall)

Inlining methods in ArduinoCoreServiceImpl (part 10: PlatformInstall, PlatformDownload, PlatformUninstall, PlatformUpgrade, PlatformSearch)

This change is quite challenging because it implements a bidirectional
streaming service. The gRPC implementation is slightly simpler, BTW the
command-line requires a bit of streams fiddling to get the same
behaviour as before because now:

 * the Monitor call do not return anymore a clean io.ReadWriteCloser.
 * the call to srv.Monitor is blocking until the port is closed or the
   context is canceled.

Added helpers to get download progress.

Added helpers to get init progress.

The BoardListWatch RPC call has been converted into a method of the gRPC
server implementation.

This commit boasts an helper method to convert a gRPC streaming response
into a channel.

This patch inlines BoardList and expands the gRPC API to provide the
same output given to the cli.

This commit is composed of many parts:

* ArduinoCoreServiceImpl has been made private
* A new function NewArduinoCoreService() has been added to instatiate a
  new service.
* The service is created at CLI startup and sent down the initialization
  chain where needed.

In this commit only the BoardListAll command has been ported to prove
the feasability of the change.

This change avoids the ugly 'convertErrorToRPCStatus' helper method
since the gRPC server automatically recognize if the error implements
the `GRPCStatus() *Status` interface method.