oneclickexport: add code host configs export functionality

Created by: sashaostrikov

One thing: current design of how ExportRequest is composed and processed (very ugly and straightforward ATM) will be changed when new Processor types are added.

I'll resolve merge conflicts after parent PR is merged. Conflicts resolved

Depends on https://github.com/sourcegraph/sourcegraph/pull/39813

Closes https://github.com/sourcegraph/sourcegraph/issues/39618

Test plan

Separate test case for code host config export is added. Cumulative test for complete export of everything currently supported is added.