build: T6855: make the custom APT entry and key syntax more flexible

[dmbaturin]

Change Summary

Right now, our custom repository and APT key options have a few shortcomings:

• APT entries are monolithic, so Debian distribution has to be updates in all entries when we switch to a new base distro.
• A lot of custom repos only have main component, but we force the config author to specify it.
• Configs may only include APT key file paths, so there's no way to distribute a self-contained flavor that uses custom repos with custom keys.
• Renames the oddly-sinfular custom_apt_key config option to custom_apt_keys.

This PR:

• Makes custom APT entry syntax in build configs more flexible and allow specifying individual components instead of monolithic sources.list strings.
• Allows embedding APT keys in configs.
• Generates Debian source repository entries for all repositories.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build scripts and configs.

Proposed changes

[additional_repositories.some-repo]
  # Optional, no default
  architecture = "amd64"

  # Optional, defaults to "main"
  components = "main contrib"

  # Optional, defaults to false
  no_source = true

  # Optional, defaults to build_config['debian_distribution'
  distribution = 'bo'

  # Mandatory
  url = 'https://example.com/some-repo/debian'

  # Optional
  key = ''' <Base64 encoded APT key data> '''

The new APT key mechanism can also be used at the config level rather than at the repository level:

[[custom_apt_keys]]
  name = 'foo'
  key = '<Base64 data>'

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title