Reduce duplication of curl options computation

Some curl option are request-specific, but not node specific: e.g. rpc
timeout, etc. The patch changes the HttpClientRequest type so that we
can pre-seed such options, instead of rebuilding the list in each
individual request execution.

Note: this was sent before and LGTMed, but on a different codebase, so
resending an updated version.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>