This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
This is based on `ssl_mail_client` from mbedtls.
The client also adds functionality to send attachments.
Signed-off-by: Vikram Dattu <vikram.dattu@espressif.com>