[New CDot Nav] Follow-up: Move `parseAndConvertSubscriptionJson` to a util

As a part of [New CDot Nav] Add company details to billing a... (#6787 - closed) and https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7874+ a util, parseJSONAndCamelize parseAndConvertSubscriptionJson and parseAndConvertProductJson, was created that we could use elsewhere, so this issue is to move it to a helper and add tests.

See draft MR here: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8052+.