feature request: allow both plaintext and TLS connections

I switch between calling out to a few different gRPC services all the time. Some of these live locally, and some of them live remotely. Some of them operate in plaintext, and some of them use TLS. I constantly get error messages, because I forgot to either add or omit `-plaintext`. I'd love a feature (either enabled by default or behind a flag/environment variable), where grpcurl tolerates both TLS and plaintext - i.e. automatically chooses the right protocol based on the response from the server.

If this is a desired feature, I'd be happy to take a stab at coding it up.

Start by tracing grpcurl's handling of the `-plaintext` option and the connection setup used for local and remote gRPC services. Determine how the client could distinguish TLS from plaintext and define the behavior for automatic selection, including any flag or environment-variable interface. Done means grpcurl can connect to both kinds of servers without requiring the user to add or omit `-plaintext`.

Written by the indexing model from the issue text.