feat: Add timestamps to segments models

[zachaysan]

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• [x] I have run pre-commit to check linting
• [x] I have added information to docs/ if required so people know about the feature!
• [x] I have filled in the "Changes" section below?
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Changes

Adds timestamps to segments module models. Includes code to nullify the timestamps for existing records.

How did you test this code?

Checked the database changes when running locally and verified that they're properly set.

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| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/docs/ENhKSDnU4QA1YreVqh4uVBZPn2aT)) | [Visit Preview](https://docs-git-feat-addtimestampstosegmentsmodels-flagsmith.vercel.app) | | Jun 27, 2024 3:38pm | | **flagsmith-frontend-preview** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-preview/BVMW5MvimTkUZM1wYE4C4xUkmdvF)) | [Visit Preview](https://flagsmith-frontend-preview-git-feat-addtimesta-9a92e9-flagsmith.vercel.app) | | Jun 27, 2024 3:38pm | | **flagsmith-frontend-staging** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-staging/Hk5yDEvPkQPiFYPxRAiSr9BPDxeU)) | [Visit Preview](https://flagsmith-frontend-staging-git-feat-addtimesta-399507-flagsmith.vercel.app) | | Jun 27, 2024 3:38pm |

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.60%. Comparing base (bc9b340) to head (d3b6abb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4236 +/- ## ======================================= Coverage 96.60% 96.60% ======================================= Files 1194 1195 +1 Lines 39071 39094 +23 ======================================= + Hits 37745 37768 +23 Misses 1326 1326 ```

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