commands: add clean command

[mr-cal]

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Overview

Add command rockcraft clean with stubbed out TODOs. This lays down groundwork needed to refactor clean_project_environments().

There are 3 ways the clean command is used:

1. rockcraft clean <part>

• cleans a specific part
• added as a TODO to be implemented later

2. rockcraft clean --destructive-mode

• cleans the local build environment
• added as a TODO, but users shouldn't encouter this, since destructive mode is also unsupported and labeled with a TODO

3. rockcraft clean

• cleans a provider environment
• added as a TODO, but will be supported in my next PR

(CRAFT-1394)

[tigarmo]

thankss

[cjdcordeiro]

thanks for the PR.

could you please create the docs for it? and for future PRs, it is easier to stay on track if we don't merge without the corresponding docs (otherwise some poor soul will have to, later on, spend hours on digging and writing existing features :p )