Support first-class aliases for image variant tags

Problem

Bakery supports a single tagDisplayName for an image variant. When a variant needs multiple tag names—for example, the Standard variant should be published using both std and default—the current workaround is to add several hard-coded tagPatterns to the variant.

That is clunky and easy to get wrong because aliases must manually reproduce the desired combinations for version, OS, latest, and release-channel tags, including the appropriate filters.

Proposed improvement

Add first-class support for variant tag aliases, for example:

variants:
  - name: Standard
    extension: std
    tagDisplayName: std
    tagAliases:
      - default
    primary: true

The aliases should participate anywhere the canonical variant tag is used, while preserving the existing primary-variant and primary-OS filtering behavior. This should create equivalent tags such as:

• 1.0.0-ubuntu-22.04-std
• 1.0.0-ubuntu-22.04-default
• default for the latest primary variant

The naming and exact configuration shape are open to discussion; the goal is to avoid duplicating tag-pattern logic for common aliases.