Fix Custom Cost window and step calculations

[nik-kc]

What does this PR change?

• Fixes the calculations used to determine windows and accumulations for Custom Cost queries.
• Exposes these fixed calculations for use by KCM.

Does this PR relate to any other PRs?

• KCM PR.

How will this PR impact users?

• Custom Cost queries for windows of e.g. 48h will show matching and complete time series and totals data.

Does this PR address any GitHub or Zendesk issues?

• Closes BURNDOWN-368.

How was this PR tested?

• Tested locally against Kubecost Datadog data:
  • Verified that /customCost/total and /customCost/timeseries` responses contained matching and complete cost data for windows that use both hourly and daily data stores:
    • 24h
    • 48h
    • 1d
    • 3d
    • 7d
    • 30d

Does this PR require changes to documentation?

• Nope.

Have you labeled this PR and its corresponding Issue as "next release" if it should be part of the next OpenCost release? If not, why not?

• v2.2

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Name	Status	Preview	Comments	Updated (UTC)
opencost	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 27, 2024 8:37pm

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
5.6% Coverage on New Code
5.3% Duplication on New Code

See analysis details on SonarCloud