Stakeless gauge checkpointer: generic cost gauges

[jubeira]

Description

This PR makes the stakeless gauge checkpointer more future proof and flexible, while making it compatible with the new avax gauges.

Summary of the changes:

• Introduce the notion of gauge costs for any gauge type (not only Arbitrum). The checkpointer is prepared in case legacy (mostly all existing non-arbitrum gauges) not implementing the interface to retrieve the cost, and defaulting to 0 in that case.
• Add more methods to checkpoint in a flexible manner: by a given list of types (which supersedes the function to checkpoint a single type).
• Moved the argument checks to the external / public functions. Before this change some checks were implicit (mostly for single gauge checkpoints); now all the functions are protected at the entry level, and internal functions just work with validated inputs.

Type of change

• Bug fix
• New feature
• Breaking change
• Dependency changes
• Code refactor / cleanup
• Documentation or wording changes
• Other

Checklist:

• The diff is legible and has no extraneous changes
• Complex code has been commented, including external interfaces
• Tests are included for all code paths
• The base branch is either master, or there's a description of how to merge

Issue Resolution

N/A

[EndymionJkb]

Have some suggestions for a little more refactoring

[jubeira]

Thanks for the review! Could you please take one more look at the latest changes?

[EndymionJkb]

Looks good now. (One small comment nit: a pet peeve :)