[breaking] Refactor initialization steps

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
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• 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?

A big refactoring of the initialization steps run by the CLI both when run "one shot" or in daemon mode.

• What is the current behavior?

Currently the gRPC Init function that creates and initializes a new CoreInstance to handle platforms and libraries that are both installed and installable returns an error and fail completely if even just one installed platform or library fails to load for any reason.

This issue breaks completely the CLI or any gRPC consumer when the initialization step fails because no CoreInstance is created, thus preventing any other further operation.

• What is the new behavior?

The Init gRPC function has now been split into two separate functions, Create to create a new CoreInstance and folders necessary to correctly execute the CLI, and Init that loads indexes, platforms and libraries.

The Create function will only fail if the creation of the necessary folders will fail, in any other case a valid CoreInstance is returned.

The Init function has been greatly improved, now failure to load a single platform or library doesn't completely stop the loading, but a message is sent to the client to notify the failure, the client will decide how to handle the error.

The CLI will print an error to the user whenever a platform or library fails to load but execution won't stop, it will only fail if the platform or library is needed to run successfully the command.

The Rescan gRPC function has been removed completely in favour of Init.

UpdateIndex and UpdateLibrariesIndex gRPC functions now don't implicitly reload the indexes, it has to be done explicitly with the Init function.

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

Yes. Further information to handle the breaking change can be found in the UPGRADING.md file.

• Other information:

None.

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[silvanocerza]

Rebased after #1278 merge.