Add DORA 4 lead time API frontend fixtures

What does this MR do?

Adds three new frontend fixtures that represent the return value of the DORA 4 Lead Time API.

Implementation

These fixtures are similar to the existing DORA 4 Deployment Frequency API fixtures, so I was able to restructure the existing Deployment Frequency fixture code to support both cases. This restructuring will also allow future DORA 4 metrics to reuse this code in a similar fashion.

Fixture content

tmp/tests/frontend/fixtures-ee/api/dora/metrics/daily_lead_time_for_changes_for_last_week.json

[
  {
    "2015-06-30": 86400.0,
    "date": "2015-06-30",
    "value": 86400.0
  },
  {
    "2015-07-01": 432000.0,
    "date": "2015-07-01",
    "value": 432000.0
  },
  {
    "2015-07-03": 604800.0,
    "date": "2015-07-03",
    "value": 604800.0
  }
]

tmp/tests/frontend/fixtures-ee/api/dora/metrics/daily_lead_time_for_changes_for_last_month.json

[
  {
    "2015-06-25": null,
    "date": "2015-06-25",
    "value": null
  },
  {
    "2015-06-30": 86400.0,
    "date": "2015-06-30",
    "value": 86400.0
  },
  {
    "2015-07-01": 432000.0,
    "date": "2015-07-01",
    "value": 432000.0
  },
  {
    "2015-07-03": 604800.0,
    "date": "2015-07-03",
    "value": 604800.0
  }
]

tmp/tests/frontend/fixtures-ee/api/dora/metrics/daily_lead_time_for_changes_for_last_90_days.json

[
  {
    "2015-06-01": null,
    "date": "2015-06-01",
    "value": null
  },
  {
    "2015-06-25": null,
    "date": "2015-06-25",
    "value": null
  },
  {
    "2015-06-30": 86400.0,
    "date": "2015-06-30",
    "value": 86400.0
  },
  {
    "2015-07-01": 432000.0,
    "date": "2015-07-01",
    "value": 432000.0
  },
  {
    "2015-07-03": 604800.0,
    "date": "2015-07-03",
    "value": 604800.0
  }
]

Steps 👣

1. !58442 (merged)
2. !58708 (merged) (this MR)
3. !57872 (merged)

Related to #250329 (closed)