Fix dependent diff updates being rejected

[cycraig]

Description of change

Fixes the DiffChain rejecting DiffMessages that update document sections added in previous diffs. From the example given in #593: adding a service in a diff update then editing or removing it in another diff update will result in the latter update being rejected/discarded by the DiffChain during resolution.

Thank you to @PhilippGackstatter and @abdulmth for identifying and investigating the issue!

This also flattens the properties of DocumentMetadata serialization (this ensures unhandled fields are captured during deserialization), which may lead to breaking changes if any existing user-defined custom fields conflict with hardcoded field names.

WARNING: this may result in resolutions of IOTA DID Documents that contain diff updates being different after this is merged.

Links to any relevant issues

Fixes #593

Type of change

• 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

Existing and new tests pass, including examples.

Change checklist

• 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

[PhilippGackstatter]

Unfortunately, I can't confirm it's working on my side. The example from #593 is printing the expected result, but that seems to be a coincidence. No matter how many diffs I publish in try_from_index_with_document,

index.get(
      this
        .current_message_id()
        .unwrap_or_else(|| current_document.message_id()),
    ).unwrap().len()

is always 1.

For instance, if I run an example that adds service-1, deletes service-1 and adds service-2, the service property is still empty when being resolved, even though I confirmed three diffs are being published.

[cycraig]

Thanks for catching that! Turns out there's an entirely unrelated bug to what this PR fixes and I tested insufficiently (multiple times).

Almost all DiffMessages fail validation after deserialization now because of the change in #598. It appears that updates encoded in DiffIotaDocument change after (de)serialization to JSON, which means the signature validation fails since when it's serialized a second time the JSON changes. I'll open another PR to address this...

Current unit tests failed to catch this since they instantiate DiffMessages directly rather than deserialising them from the Tangle, and we don't have asserts in the examples so I just assumed they were working.

[cycraig]

Opened #611 to fix the deserialisation bug, will re-evaluate this PR once that's merged. Merged.

[cycraig]

Seems to be fixed, I manually checked the examples this time!

[PhilippGackstatter]

Looks good to me, can confirm my example is also working now 👌! Thank you very much for fixing this!

[PhilippGackstatter]

I agree, then we can remove this:

identity.rs/identity-account/src/updates/update.rs

Lines 264 to 267 in 630a61e

	ensure!(
	state.document().service().query(&service_url).is_some(),
	UpdateError::ServiceNotFound
	);

Doesn't need to be in this PR.