Lambda.Context should be a struct

[fabianfett]

Lambda.Context is currently a class. But it is just a wrapper around values. This should be a value type.

[fabianfett]

Basically we need the changes from here, minus the addition of Baggage.

https://github.com/swift-server/swift-aws-lambda-runtime/pull/167/files

[fabianfett]

Thanks @jareyesda for fixing this! And congrats to your first open source contribution!