Support merging schemas for OpenApiOperations sourced from metadata

[captainsafia]

Related to https://github.com/dotnet/aspnetcore/issues/45265 and https://github.com/dotnet/aspnetcore/issues/45307

For scenarios where people provide an explicit schema definition in their WithOpenApi call, we should merge or favor that definition over the one generated by default.

Typical scenario is people wanting to fix incorrectly produced schema for types like Stream and Guid.

[warappa]

Still having this problem on .NET 8.0.1