Aggregating ITypeShapeProvider

[AArnott]

Consider an ITypeShapeProvider implementation that aggregates results from a list of other ITypeShapeProvider instances.
This aggregator would take a list of other instances and return the first shape from the list of providers that returns a non-null result.
It may include caching of the result for faster repeat calls.

This would enable combining multiple shape providers and querying them in a unified manner.
The scenario I'm thinking about is of an RPC system that may have access to several source generated ITypeShapeProvider instances, and may also want to fallback to the reflection-based ITypeShapeProvider when no source generated one is known.

Example usage

ITypeShapeProvider provider1 = ...;
ITypeShapeProvider provider2 = ...;
ITypeShapeProvider aggregatedProvider = new AggregatedTypeShapeProvider(new[] { provider1, provider2 });

var shape = aggregatedProvider.GetShape(typeof(MyType));

[eiriktsarpalis]

Here's the relevant concept in STJ: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.metadata.jsontypeinforesolver.combine?view=net-9.0