CLI improvements proposal: positional argument

[happyRip]

Summary

Closes #56

Changes

• Add an optional positional source argument (pkg/arg)

• Print only source-related flags with those commands

  • Removed prefixes from ttnv3 flags; to be followed with other sources

• Split application and devices commands FlagSets

• Improved ttnv3 config by utilising flags that save value to variables directly

Testing

local testing

• [x] The Things Stack
• [x] Chirpstack v3
• [ ] ttnv2

Regressions

I do not expect any.

Notes for Reviewers

• If we were planning to add more commands in the future, FlagSet splitting could be upgraded to a more sophisticated solution. Currently it's pretty simple.

• It was suggested to use generated commands like tts-devices ETC, but I wanted to propose this solution which seems to work well and is pretty simple

• When pflag is used to set ActiveSource the value is set after initialisation, so the flags cannot be separated easily (during FlagSet() function call)

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [ ] Documentation: Relevant documentation is added or updated.
• [x] Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.

[adriansmares]

Is this flag magic really necessary? Why don't we have commands per source with their own flag sets?

@happyRip I think this actually may be better than our current hack. We would need to generate dynamically an application / device sub command per source, but I think it would indeed work better here.

[adriansmares]

What is the status here - is this blocked on anything ? Does it need any technical decision ?

[happyRip]

Subcommands approach was merged, so this PR can be closed.