[Console] application/command as text/xml/whatever decoupling
merged branch jfsimon/console-descriptor (PR #7454)

This PR was squashed before being merged into the master branch (closes #7454).

Discussion
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[Console] application/command as text/xml/whatever decoupling

This PR removes description generation from `Command`, `Application` and `InputDefinition` classes and delegate it to specialized descriptor classes, making it dead simple to add new output formats.

**Maybe this could include other commands, like `router:debug` or `container:debug` (see #5740)?**

- [x] Add a `DescriptorProvider` which uses `DescriptorInterface` objects to describe things.
- [x] Add `txt` descriptors.
- [x] Add `xml` descriptors.
- [x] Add `json` descriptors.
- [x] Add `md` descriptors.
- [x] Remove obsolete methods.
- [x] Repair tests.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6339

Commits
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51512e6 [Console] application/command as text/xml/whatever decoupling