Rework Quantity parsing

[Techassi]

Our custom Quantity parsing doesn't match K8s behavior completely. This issue tracks the improvement of our parsing. This might break tests.

Parsing spec: https://kubernetes.io/docs/reference/kubernetes-api/common-definitions/quantity/

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This issue was raised after an internal arch meeting where the code style around resource specifications was discussed.

### Acceptance criteria
- [ ] tests are added that cover the current mismatch in parsing
- [ ] the parsing is fixed and in sync with k8s parsing
- [ ] quantity format document in the documentation is updated

See

• https://github.com/orgs/stackabletech/discussions/60
• MemoryQuantity parsing fails when unit is missing community#61