Enhanced TLS configuration

[ianbotsf]

(split from #661)

Add additional APIs to configure HTTP engine TLS context.

This may be at the common HttpClientEngineConfig level or at individual engine config level depending on what makes sense.

Possible knobs:

• min TLS version (handled in TLS Configuration #661)
• CA certs (trust manager)
• Cipher preferences
• Certificate pinning
• Host name verifier

Not every engine will support all of these or even if they do may make it difficult to map a common API to.

After these changes are made, coordinate updates to the Developer Guide to clarify how to configure TLS.

Once certificate management is available, complete the HTTP test suite to verify that TLS min versions are working correctly on local test servers with self-signed certificates (see TODO in ConnectionTest.kt).