Make FlowHandler.context a typed dict

[emontnemery]

Proposed change

Make FlowHandler.context a typed dict

Also, add the type of flow context as a generic to FlowHandler and FlowManager

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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