MINOR: httpclient: add an "https" log-format

MINOR: httpclient: add an "https" log-format

Add an experimental "https" log-format for the httpclient, it is not
used by the httpclient by default, but could be define in a customized
proxy.

The string is basically a httpslog, with some of the fields replaced by
their backend equivalent or - when not available:

"%ci:%cp [%tr] %ft -/- %TR/%Tw/%Tc/%Tr/%Ta %ST %B %CC %CS %tsc %ac/%fc/%bc/%sc/%rc %sq/%bq %hr %hs %{+Q}r %[bc_err]/%[ssl_bc_err,hex]/-/-/%[ssl_bc_is_resumed] -/-/-"