[Task] Make DID Document optional

[eike-hass]

Description

Make the DID document field in On-Chain Identities optional. For now let's only consider the breaking change in Move and minimal changes to the library to guarantee reliable operation.

Motivation

Prepare On-chain Identities for use cases where DID is unnecessary or optional.

Resources

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To-do list

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• Item one
• Item two
• Item three

Change checklist

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• The feature or fix is implemented in Rust and across all bindings whereas possible.
• The feature or fix has sufficient testing coverage
• All tests and examples build and run locally as expected
• Every piece of code has been document according to the documentation guidelines.
• If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
• If the feature is not currently documented, a documentation task Issue has been opened to address this.