Remove remaining std requirements rust oak runtime

[blaxill]

New plan, deviate slightly from the cpp runtime to avoid porting dependencies:

• make protobuf configuration compilation-optional on std. This configuration is used by the tests, and is how the cpp runtime configures the enclave, but to avoid porting a protobuf library to no_std we can pass in a rust native configuration structure. The structure can be created by a std using rust library linked into the host asylo code (outside of the enclave.)
• abstract gRPC into an enclave abstraction layer
• make oak_abi no_std compatible Make oak_abi no_std compatible  #638
• additional items placeholder

Alternative original plan, port the remaining crates and become a drop in replacement for the cpp runtime (also requires porting nodes):

• protobuf - Currently using rust-protobuf, perhaps move to prost which is closer(perhaps?) to supporting no_std - Investigate making rust-protobuf no_std compatible  #550
• wasmi - no_std support is currently broken due to a dependency. Fix is ready but not on cargo yet
  • wasmi dep num-bigint 0.3 release map with no_std support Tracking issue for num-bigint 0.3 rust-num/num-bigint#128
  • wasmi issue no_std support issue Can't build in no_std environment wasmi-labs/wasmi#204

[blaxill]

Closing as we are no longer definitively targeting no_std.