Implement asio-ssl layer with three classes in asio::ssl::mbedtls:
* context -- replaces SSL_CTX, used mainly as a container to options,
certs, keys
* engine -- replaces SSL, implements the actual mbedtls operations
* bio -- implements openssl BIO specifically tailered to mbedtls and
its asio usage
Further updates:
* asio: Used shared_ptr<> for bio pairs
* asio: Add error checks to mbedtls-bio
* asio: Address potential ssl-context ownership issue
* asio: Address potential bio-engine ownership issue
This port employs IDF port of OpenSSL for most common features, others
are discouraged or not supported. The port also introduces several stubs
for OpenSSL functions which ASIO needs to get compiled and linked.
Upstream ASIO supports WolfSSL as SSL/TLS stack, as well, which is
another option for SSL support in ASIO on ESP platform.
