feat: let callers set an explicit versionTime on create/update

[stormer78]

What

Adds an optional version_time to CreateDIDConfig and UpdateDIDConfig so callers can set an explicit versionTime on the log entries they create. Default None preserves today's behavior exactly (the lib stamps now()).

let cfg = CreateDIDConfig::builder()
    .address(addr).authorization_key(k).did_document(doc).parameters(p)
    .version_time(Utc::now().fixed_offset() - Duration::hours(1)) // optional
    .build()?;

Why

versionTime serializes at second granularity (SecondsFormat::Secs) and verify_version_time requires each entry to be strictly greater than the previous and not in the future. create_did/update_did hardcoded create_log_entry(None, …) → now(), so a caller creating multiple entries in quick succession (e.g. a service that does create-then-update back-to-back) gets two same-second entries that serialize identically and make the DID unresolvable ("versionTime must be greater than previous").

Notably this passes every in-memory test (the monotonic check runs on the full-precision parsed DateTime) but fails once the log is serialized + re-resolved — which is exactly how it bites in practice.

Rather than have the lib invent timestamps (bumping to prev+1s trips the not-in-the-future check; widening serialization precision would change the canonical/hashed form and break existing dids), the clean fix is to let the caller decide: a batch of updates can backdate + space the entries (e.g. now−1h, now−1h+1min) so the log stays strictly increasing and in the past. The low-level DIDWebVHState::create_log_entry already took Option<DateTime<FixedOffset>>; this just surfaces it on the high-level builders.

Scope

• version_time is honored on the single-entry operations: genesis create_did, the standard update_did, and the migrate path.
• The deactivation path can append two entries (pre-rotation teardown + final); one caller-supplied value can't keep them distinct, so it keeps the default per-entry now() stamping (unchanged).

Test

create_then_update_with_spaced_version_times_validates — a genesis backdated to now−1h plus an update at now−1h+1min validates end-to-end (report.truncated.is_none()), the sequence that fails with same-second defaults. All existing tests unchanged and green.