Make scheduling policy used in schedule_recurrent configurable

Add support for a configuration option "scheduling_policy" in the config
file loaded by the 'swh scheduler schedule-recurrent' command. This
config entry allows to specify the scheduling policies used by the
schedule-recurrent tool, instead of having them hardcoded in the source
code.

A visit type policy config entry should have at least a 'weight' value
for each policy.

Default values are unchanged.

Eg.:

scheduling_policy:
  git:
    - policy: already_visited_order_by_lag
      weight: 55
      tablesample: 0.5
    - policy: never_visited_oldest_update_first
      weight: 45
      tablesample: 0.5

Note: there may not be configuration entries for all visit types, but if

a visit type policy is configured, the config entry should be complete
(in other words, the merging of the configuration with the default
values is only done at first config level).