Add templates submodule for libcrypto

Libcrypto stubs (e.g., ec_override.c which come from aws-c-common submodule) can now be submoduled from: https://github.com/awslabs/aws-verification-model-for-libcrypto

Start by inspecting the libcrypto stubs mentioned in the issue, including ec_override.c, and review how the existing aws-c-common submodule is wired. Compare the available templates in the linked aws-verification-model-for-libcrypto repository; done means the libcrypto stubs are sourced through the new templates submodule.

Written by the indexing model from the issue text.