* op-challenger: Introduce --prestates-url to specify prestates source for all game types at once.

* op-challenger: Add required config validation for asterisc kona.

Allow specify both prestate and prestates url to allow overriding. Prestate takes priority.

* op-challenger: Introduce vm-flag to make default with overrides pattern easier.

* op-challenger: Add vm type info to override flags.

* op-challenger: Support file: URLs to download prestates from.

Makes it possible to support multiple prestates without needing a HTTP server.