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

[filzrev]

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

Currently Microsoft.DocAsCode.App expose Docset.Build 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 <InternalsVisibleTo Include="docfx" /> settings and modify affected classes.