Add TLS and DTLS support

[mpilquist]

This PR adds the fs2.io.tls package, consisting of the types: TLSContext, TLSSocket (for TCP), and DTLSSocket (for UDP). This was originally based off fs2-crypto by @pchlupacek @AdamChlupacek and @mraulim but I ended up with a different approach to the internal TLSEngine class.

Usage examples are in the TLSSocketSpec and DTLSSocketSpec classes.

Note: requires building on Java 11+.

Fixes #1696.

[tpolecat]

requires building on Java 11+

Will it also require downstream programs to build on 11, and/or applications to run on 11? I don't really care either way but Skunk users will want to know.

[mpilquist]

@tpolecat No, TLS stuff will still run on Java 8+, though no DTLS support then.

[mpilquist]

I added some docs on fs2-io to the guide -- treatment of non-networking sections is pretty light but better than nothing. This PR is ready for review.

[pchlupacek]

@mpilquist seems good to me. Excellent work, thanks.

[tpolecat]

🎉 this is great! I will give it a go with Skunk in the next few days and let you know how it goes.

[SethTisue]

congrats, you are the first repo in the entire Scala community build to drop support for building on JDK 8

(which is fine, it's not a problem, I just wanted to note this historic occasion)