Initialize PjRt plugin

[joelberkeley]

We don't do this

struct PJRT_Plugin_Initialize_Args {
  size_t struct_size;
  PJRT_Extension_Base* extension_start;
};
PJRT_DEFINE_STRUCT_TRAITS(PJRT_Plugin_Initialize_Args, extension_start);

// One-time plugin setup. Must be called before any other functions are called.
typedef PJRT_Error* PJRT_Plugin_Initialize(PJRT_Plugin_Initialize_Args* args);