cli 2.0: Add commands management

[czoido]

Changelog: Feature: Add commands management for cli 2.0.
Docs: omit

First PR to introduce the changes of #7170

Now the commands are in separate python files, and have decorators to add the group and formatters. The formatters can go in the same python file that the command. To define a new command the python file should be added to the cli/commands folder and be defined similar to this:

def output_search_cli(info, out):
    ...

def output_search_json(info, out):
    ...

@conan_command(group="Consumer commands", formatters={"cli": output_search_cli,
                                                      "json": output_search_json})
def search(*args, conan_api, parser, **kwargs):
    """
    Doc string that will appear when using --help
    """
    ....
    return info

The command always return a info that will be used by the formatter to show the final output of the commands depending on the --output argument (that argument has cli value by default)

To test this the CONAN_V2_CLI environment variable must be defined.

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• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[memsharded]

I like very much this. A few comments and points for discussion, please check.

[jgsogo]

Some comments about the implementation, I think we can simplify it a little bit. Maybe a PR would have been easier 😅

[memsharded]

I would say this is good as-is for merging, other improvements and fixes will follow when more commands are added, or with the new conan_api.

[jgsogo]

Let's merge and start building on top of this 🎉