Allow log that changes version midway through

[brianorwhatever]

Implement per-entry protocol detection and process each contiguous segment with the matching resolver. When a version change is detected, finalize the current resolver’s output and continue resolving the next segment with the new version’s resolver.

Something like this:

1. Introduce a helper in src/method.ts to compute protocol version for a single DIDLogEntry.
2. Update resolveDIDFromLog to iterate over log entries, splitting the log whenever parameters.method changes.
3. For each segment, call the appropriate resolver (v0_5.resolveDIDFromLog or v1.resolveDIDFromLog) and keep track of the most recent doc, meta and did.
4. After processing all segments, return the result from the last resolver invocation.
5. Add tests demonstrating that a log beginning with did:webvh:0.5 entries followed by did:webvh:1.0 entries resolves correctly.
6. Document this new behavior in README.md.

[swcurran]

@andrewwhitehead -- does the Python implementation have support for this type of change?

A new specification version will have to indicate what has to happen when changing versions from the previous one. For example, from v0.5 to v1.0 -- the removal of the weight, and the redefinition of prerotation that now must be done on every update. I think those are right.