[native_assets_cli] Support adding new assets in hook/link.dart

[dcharkes]

This needs to be supported downstream:

• [native_assets] The native assets mapping should use kernel concatenation in AOT sdk#55377
• [native_assets] Use kernel concatenation instead of passing in asset-mapping into kernel compilation flutter/flutter#146270

After we support adding assets in link hooks, we should update the doc comments on link.

The current workaround is to always add assets in hook/build.dart. @mosuem doesn't this workaround actually lead to duplicate assets? We would be "overriding" assets from the build hook with the link hook output. (Maybe I should prio the two above issues and the ones blocking that.)

[mosuem]

Adding new assets in link.dart is not a priority I would say - but it would need some checking to avoid duplicating assets, or a policy on how to resolve duplicate assets.