AVRO-3893: [csharp] Avoid per-call closure allocation in ObjectCreator.FindType

[iemejia]

What changes were proposed in this pull request?

ObjectCreator.FindType passed an inline lambda to ConcurrentDictionary.GetOrAdd:

return typeCacheByName.GetOrAdd(name, (_) =>
{
    ...
    if (TryGetIListItemTypeName(name, out var itemTypeName)) { ... }   // captures `name`
    ...
});

The lambda discards the key parameter ((_)) and instead captures the local name. Capturing a local forces the C# compiler to allocate a new display-class closure (and delegate) on every call — including cache hits, since the factory delegate is constructed as an argument regardless of whether GetOrAdd invokes it. As reported in AVRO-3893, this accounted for ~10% of allocations on a deserialization hot path using PreresolvingDatumReader.

How was this patch fixed?

• Extract the value factory into a private FindTypeUncached(string name) method.
• Store a single cached Func<string, Type> findTypeFactory delegate, created once in the constructor, and pass it to GetOrAdd.
• The factory uses its name parameter (the cache key supplied by GetOrAdd) instead of a captured local, so no closure is allocated per lookup.

ObjectCreator is a shared singleton (ObjectCreator.Instance), so the factory delegate is effectively allocated once for the process. Behaviour is unchanged; the CA1031 suppression is retargeted from FindType to the extracted FindTypeUncached.

How was this patch tested?

• dotnet build of Avro.main succeeds with 0 warnings (confirming the retargeted suppression).
• Avro.test Specific/ObjectCreator suites pass: 96/96 across net6.0, net7.0, and net8.0.