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

We introduced a `WriteAdditionalFiles` API, and it is implemented to write `ConfigurationSchema.json`: https://github.com/microsoft/typespec/blob/c84f7564f66f5c620708a0322f1b062521015865/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/ScmCodeModelGenerator.cs#L55

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.