decouple config creation from VerifyingKey::read

[Liuhaai]

This PR aims to simplify the loading of ProvingKey and VerifyingKey.
Since config from VerifyingKey::read is only used in keygen_vk, the config creation is separated as create_config func which is called immediately after create_domain func in keygen_vk.
This refactoring will remove the requirement of params and concrete circuit when deserializing the ProvingKey and VerifyingKey from bytes. e.g. https://github.com/privacy-scaling-explorations/halo2/pull/206/files#diff-073662488bf261a70d2143e3a1da2e9e61511216975fbdb13833c8253b819df8R146

[CPerezz]

Approved to run the tests.

I just skimmed through this and did not look bad.
But want to take a deeper & closer look next week.

[han0110]

I think this makes the cs always empty in vk, the ConcreteCircuit generic would be required as long as cs is not serialized.