feat(group): add escalation links skeleton between groups

[MyvTsv]

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

Lay the groundwork for an escalation hierarchy between groups: a group will declare the groups it can escalate to, per entity, with a link type that drives how the link is displayed and whether it is replicated in sub-entities. This PR only adds the skeleton; the logic will come in follow-up PRs.

Changes

• New Group_Link relation (glpi_plugin_moreoptions_groups_links): source group, destination group, entity, link type, creation/update dates. A pair of groups can only be linked once per entity.
• New "Escalation" tab on groups (empty template for now).
• Link types implemented with the Strategy pattern.
  • basic: link restricted to its entity.
  • inherited: link replicated in the sub-entities.
  • none: removal of an inherited link in a sub-entity, which also stops its replication below.
• Group_Link::getLinksForEntity() is declared and documented, but not implemented yet.