Make StateMetadataDocument public

[PhilippGackstatter]

Description of change

We currently get warnings that StateMetadataDocument::{into_iota_document,unpack} are unused when running cargo check --no-default-features.

This PR makes StateMetadata public to get rid of those warnings. Another approach would have been to feature-gate everything state metadata related behind the client feature. The reasoning for making it public is that StateMetadata can be used with clients other than iota-client and there's no inherent reason why it has to be tied to that specific client. We may also want to use it from the Wasm bindings without adding a dependency to iota-client in order to decode the state metadata of an Alias Output resolved by, say, iota.js.

Eq was also derived for StateMetadataDocument to satisfy a clippy lint.

Links to any relevant issues

n/a

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

None.

Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[olivereanderson]

LGTM.

Remember to address my comments after #1086 has been merged.