Typed context

[JordonPhillips]

This adds a typed context bag, which is essentially a dict with some extra type forwarding built in. It removes the need to pass around a bunch of dict[str, Any] all over the place without sacrificing the ease that doing so provides.

The intended use of this is to have statically defined typed properties, like so:

SERVICE_ID = PropertyKey(key="service_id", value_type=str)
CONFIG_PROPERTY = PropertyKey(key="config", value=Config)

...

context.properties[SERVICE_ID] = "my-service"

...

# After plugins have modified config
context.properties[CONFIG] = config

...

service_id = context.properties[SERVICE_ID]
config = context.properties[CONFIG]
endpoint = endpoints.get(service_id, {}).get(config.region)

The property keys can then be shared so that nobody messes up the key or its typing, like we do in the code generator for symbol properties.

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[alextwoods]

Nice! I like this. I'll put out an update to the UserAgent to use a typed property once this is merged :-)

[JordonPhillips]

Had to rebase