ostree-repo-pull: add options to configure retry behavior

ostree-repo-pull: add options to configure retry behavior

This introduces the "retry-all-network-errors" option which
is enabled by default. This is a behavior change as now
ostree will retry on requests that fail except when
they fail with NOT_FOUND. It also introduces the options
"low-speed-limit-bytes" and "low-speed-time-seconds these"
map to CURL options only at the moment. Which have defaults
set following librepo:
https://github.com/rpm-software-management/librepo/blob/7c9af219abd49f8961542b7622fc82cfdaa572e3/librepo/handle.h#L90
https://github.com/rpm-software-management/librepo/blob/7c9af219abd49f8961542b7622fc82cfdaa572e3/librepo/handle.h#L96
Currently these changes only apply when using libcurl.
Finally this change adds a final option that affects all
backends to control the max amount of connections of the
fetcher "max-outstanding-fetcher-requests".