Add Flag typings to feafea that can be used elsewhere

[simonlin527]

Adds TypedDict definitions of what a flag should look like with optional and mandatory fields. These are helpful so we can add typings to flags in places where feafea is used.

[github-actions]

📊 Test Coverage Report

✅ Coverage Status: PASSED 🎉

Current Coverage: 100.00% (Required: 100%)

📋 Detailed Coverage Report

Name                     Stmts   Miss Branch BrPart    Cover   Missing
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src/feafea/__init__.py     583      0    256      0  100.00%
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TOTAL                      583      0    256      0  100.00%

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[msmans]

N/A as these are