support/contractevents: Properly parse asset topic, add event generator.

[Shaptic]

What

This PR has three components:

1. It fixes a misunderstanding in event parsing in which assets were assumed to be encoded in SEP-11 <code>:<issuer> format, whereas they're actually encoded as raw XDR separated by a : byte
2. It adds more tests for various parts of the library
3. It adds an exported GenerateEvent function which lets external packages generate arbitrary event fixtures for testing without worrying about packing the XDR correctly

Why

Respectively,

1. This is a bug when using this on real events and the new TestRealXdr() proves this
2. For stability, of course
3. Related issues like services/horizon: Add account_credited/debited effects for SAC events. #4806 can benefit from creating events w/o needing to understand their structure

See stellar/rs-soroban-env#727 for recommendations on improving (1).

Known limitations

n/a