Pass arguments instead of sharing globals in `@sourcegraph/code-host-integration`

Context

The following discussion from !16556 (merged) should be addressed:

• @pslaughter started a discussion:

  thought: What if integration.bundle.js exposed a function for us to pass these arguments, instead of sharing these global variables? This isn't something we need to address in this MR, but I'd like to create a follow up issue for this.