Support eHSM as an alternative KMS solution

[syan10]

Dear maintainers,

We've developed an SGX-based Cloud KMS (called as eHSM), eHSM is a cloud service to provide functionalities to manage keys and secrets by fully leveraging Intel SGX capability. It based on SGX SDK not LibOS, which has smaller TCB (Trusted Computing Base) and thus enhanced security. more details please see the follwoing link: https://github.com/intel/ehsm.git

we're investigating is it possible to upstream it into the cosign as an alternative KMS solution, if so, could you provide some guide or wikis to show how to do it? Thanks.