code-intel: allow using non default http client for LSIF index upload
Created by: Strum355
Updates lib/codeintel/upload to allow taking in a custom HTTP client. Please see the related src-cli PR for rationale and library consumption example https://github.com/sourcegraph/src-cli/pull/559.
net/http.DefaultClient implements the Client interface (guaranteed by the tests that use net/http.DefaultClient) in case we don't have a need for a customized client/wrapper when using this library in other parts of the codebase outside of src-cli.
cc @efritz for