Optionally wrap TCP stream in a TLS session in TCP driver

[vruello]

Implements #202

This has been tested with the logstash tcp input plugin.

I had to make some changes (implementing TryFrom instead of From) in the subscriptions load and import/export to validate the TCP driver configuration.

The addr parameter has been renamed to host, but addr is still valid in configuration files and the database.

The TCP driver accepts the following parameters:

# - host (required): Hostname or IP Address to send events to
# - port (required): Tcp port to send events to
# - tls_enabled (optional, defaults to false): wrap the TCP stream in a TLS session.
#       Must be set for other tls_ options to take effect
# - tls_certificate_authorities (optional, defaults to undefined): Validate server certificate
#       chain against these authorities. You can define multiple files or paths.
#       All the certificates will be read and added to the trust store.
# - tls_certificate (optional, defaults to undefined): Path to certificate in PEM format.
#       This certificate will be presented to the server.
# - tls_key (optional, defaults to undefined): Path to the private key corresponding to the
#       specified certificate (PEM format).