Allow multi-base builds

[mr-cal]

What needs to get done

⚠️ Craft-application needs to allow applications to implement multi-base builds but doesn't change the default build planner.

Polish the prototype in work/CRAFT-3668/multibase-prototype to support multi-base builds as defined in ST124.

Missing work

1. Add get_base(), get_build_on_architecture() and get_build_for_architecture() to the ProjectService.
   • Application-defined services should use these instead of self._project.base and self._build_plan[0].
2. Unit tests
3. Changelog, tag and release

Why it needs to get done

To support multi-base builds in charmcraft.

Upstream work

• Add multi-bases support for charm build plans craft-platforms#73

Downstream work

• Multi-base platform support charmcraft#2008

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3719.

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