SlurmGCP. Refactor instance grouping

[mr0re1]

Refactor bulk-grouping of nodes and improve obtaining "resume_data":

• Delete unused post_main_resume_nodes plugin support;
• Refactor group_nodes_bulk as preparation for future behavior changes;
• Remove global global_resume_data;
• Add caching to to_hostnames;
• Reduce calls to to_hostnames;
• Use dataclass instead of NSDict;
• Add unit tests;

Motivation:
Current (before & after change) implementation of group_nodes_bulk is very complex. This is an attempt to deliver changes that don't affect behavior + add test coverage before working on follow up PR that will deliver behavioral changes.