[native_assets_cli] Prevent EncodedAsset and Metadata hashcode changing

[dcharkes]

Closes: #2045

We have two types of classes in the semantic API:

1. Pure views on top of a JSON.
   These classes are mirrored by a builder class.
   Objects from these classes cannot be passed from input to output in hooks.
   These classes do not override hashCode and operator ==.
   Examples: CodeConfig, BuildInput.
2. Data classes that eagerly copy all fields out of the JSON.
   These classes have a public constructor with it's constituent fields.
   Objects from these classes may be passed from input to output in hooks.
   These classes do override hashCode and operator ==.
   Examples: CodeAsset, Architecture, OS, DataAsset.

This changes Metadata and EncodedAsset to follow pattern 2 fully:

• All data is copied on construction.
• The map accessors do not allow modification.

This means the hashCode and operator == are now correct.

We could consider changing Metadata and EncodedAsset to pattern 1, but I didn't want to refactor that code in this PR. For more details:

• [native_assets_cli] EncodedAsset is not immutable but overrides hashCode #2045

[github-actions]

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coverage: 86.021% (+0.004%) from 86.017%
when pulling 694ab24 on equals-override
into 781fc27 on main.