TLS Configuration

[aajtodd]

(split into #820)

Add 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
• CA certs (trust manager) (handled in Enhanced TLS configuration #820)
• Cipher preferences (handled in Enhanced TLS configuration #820)
• Certificate pinning (handled in Enhanced TLS configuration #820)
• Host name verifier (handled in Enhanced TLS configuration #820)

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 (especially min version).

[ianbotsf]

Resolving this issue will be a breaking change. See the BREAKING: Streamlined TLS configuration post for more details.