feat: read public key from release yaml

[bryanoltman]

Description

Updates Updater to read (optional) public key from release yaml.

Type of Change

• [x] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

[codecov[bot]]

Codecov Report

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

Project coverage is 96.19%. Comparing base (6ab6972) to head (77cf072).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #168 +/- ## ========================================== + Coverage 96.15% 96.19% +0.04% ========================================== Files 28 28 Lines 3091 3126 +35 ========================================== + Hits 2972 3007 +35 Misses 119 119 ``` | [Flag](https://app.codecov.io/gh/shorebirdtech/updater/pull/168/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shorebirdtech) | Coverage Δ | | |---|---|---| | [library](https://app.codecov.io/gh/shorebirdtech/updater/pull/168/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shorebirdtech) | `97.85% <100.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shorebirdtech#carryforward-flags-in-the-pull-request-comment) to find out more.

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