Unified link converters

[Evanthx]

What was changed

Unified the Link Converter code - modelling what was done for Java and bringing into the other SDKs:
temporalio/sdk-java#3068

There are multiple encoding and decoding methods for links, all pretty close but not quite. That had me worried as it is easy for them to get out of sync. This PR should be no functional changes, but merges those methods.

I also compared the behavior across the other SDKs, and am unifying them - a few edge cases didn't quite match up. Testing was added around these edge cases.

Changes in Python SDK behavior are:

1. eventID is omitted when the event ID is 0. Zero is not a valid event ID, and the other SDKs omit the parameter; the event type alone identifies the event in that case.
2. The URL scheme is validated. This SDK matched on url.path only and never read url.scheme, so it accepted https:///nl link. Every other SDK checks.