Support asynchronous wait usercalls

[mzohreva]

No description provided.

[maxknv]

Apparently, this is the struct that needs implementation of wait() method:
https://github.com/fortanix/rust-sgx/blob/yx/new-async-usercalls_update-mbedtls/intel-sgx/async-usercalls/src/provider_api.rs#L15
And here is its counterpart in untrusted code https://github.com/fortanix/rust-sgx/blob/yx/new-async-usercalls_update-mbedtls_publish/intel-sgx/enclave-runner/src/usercalls/mod.rs#L1608

[Taowyoo]

Hi @mkaynov , I am not quite familiar with this issue.
But does this PR already cover this?

• Cancel queue + async usercall interface #404

It just needs some bandwidth from related folks to review and merge it.

Please correct me if I miss-understand something .

[maxknv]

As far as I understood, AsyncUsercallProvider requires an implementation of wait method which is not implemented on this branch