[Feature Request] Expose RunBuild.Exec API and related configs

**Is your feature request related to a problem? Please describe.**

Currently `Microsoft.DocAsCode.App` expose [`Docset.Build`](https://github.com/dotnet/docfx/blob/359ab008cba2d9891a619669e1381227541b8130/src/Microsoft.DocAsCode.App/Docset.cs#L31) API only.

and there are some limitation exists.

- `Docset.Build` API require `docfx.json` file path.
It can't directly pass configuration via code. (Because `BuildJsonConfig` is internal)
- `ConsoleLogListener` is always added on startup. It can't use custom log listeners only.

**Describe the solution you'd like**

Change `RunBuild.Exec` APIs and related configs (`BuildJsonConfig`) from internal to public.

I thought all internal APIs used by `docfx.exe` should be available also when using `Microsoft.DocAsCode.App`.
It may better to remove `` settings and modify affected classes.