Allow akashi compare to take multiple rulesets

[ryan-ph]

Why

Generally Akashi rulesets are written to contain expected changes from vN to v{N + 1}. However, in some cases, users will be incrementing their module versions by more than one (e.g. incrementing from v0.38.5 to v0.38.9).

In this case, akashi compare should still be able to validate the changes produced by this version bump only contains the changes specified in the rulesets for v0.38.{6..9}.

Proposal

Update akashi compare to take at least 1 ruleset. The CLI would then work like:

# Same as current behavior
akashi compare path/to/version1.yaml 
akashi compare  # error! no ruleset provided

# Take multiple rulesets
akashi compare path/to/version1.yaml path/to/version2.yaml

Open questions

• How should rulesets be merged? Mainly how should we merge CompareOptions that are specified for rulesets and resource changes
• How should we handle resources that change in multiple different rulesets that are provided to Akashi?