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.
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.