[http-client-csharp] `WriteAdditionalFiles` is not providing something downstream generators could filter

[ArcturusZhang]

We introduced a WriteAdditionalFiles API, and it is implemented to write ConfigurationSchema.json:

typespec/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/ScmCodeModelGenerator.cs

Line 55 in c84f756

public override async Task WriteAdditionalFiles(string outputPath)

but this extension point is an action instead of providing data - therefore for downstream generators, there are only two options: take all or reject all.
There is no such an option that a downstream generator could accept a subset in it.

Can we change this API to something like BuildAdditionalFiles which returns FileInfo[] where FileInfo is something like { filename: "", content: "" } so that downstream generators could filter them by filenames, even modify their content.