Support registering custom syscalls

[buffalojoec]

This PR adds the ability to register custom syscalls in Mollusk's test environment. This allows developers to define their own custom syscalls, register them with the test environment, then write programs that access these syscalls and test their functionality.

Since program cache entries for Agave's JIT cache require an environment parameter of type Arc<BuiltinProgram<InvokeContext<'static>>>, creating an API for developers to register syscalls was slightly convoluted.

• Add an owned BuiltinProgram<InvokeContext<'static>> field to ProgramStore directly, which can be updated when new functions are registered.
• Each time a program is added to the store, instead of calling create_program_runtime_environment_v1, create a new loader from the config and function registry of the owned loader stored on the program store. This should be functionally equivalent to what was already implemented.