[native_assets_cli] `EncodedAsset` is not immutable but overrides `hashCode`

[dcharkes]

Currently, EncodedAsset takes encoding as an argument, which is a modifiable map.

However, it also overrides hashCode and operator ==.

This is a bug.

We need to either

1. remove the operator == (and fix the tests that rely on equals) or
2. copy the map in the constructor

[dcharkes]

Ditto for class Metadata.

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.

If data from 1 needs to be passed from an input to an output (or vice versa in the native_assets_builder), then this has to be done by (a) doing it field by field, or (b) taking the json and constructing the dual mirror class with the json.

If we treat classes with maps/lists as 2, these collections need to be deep copied in the constructor (like we would parse out fields), and the accessors need to provide Unmodifiable collections.

Currently both EncodedAsset and Metadata are used as type 2. Maybe they should be moved over to be type 1.