[native_assets_cli] More connection between `Input` and `Output`

[dcharkes]

We have multiple cases where the connection between input and output could be made more clear with the programmatic API.

• buildAssetTypes -> assets ToAppBundle [native_assets_cli] Make extensions more user-friendly #2155
• linkingEnabled -> ToLinkHook [native_assets_cli] Unify Metadata with Assets #2164 (comment)
• LinkModePreference -> LinkMode

We should think of a holistic way to do this. See some suggestions by @mosuem in the linked issues.

Filing an issue to come up with a design principle and apply it everywhere where applicable in a Dart API refactoring.

(Marking as post v1.0 as it's only a Dart API change, not a JSON change.)

Edit:

• Possibly the combination of an input and output should be called Protocol. So, we'd have a BuildProtocol (possibly with an input and output field).