[ML] Add Detector config classes to protocol library

[droberts195]

This commit adds the Detector class and its dependencies to the
X-Pack protocol library used by the high level REST client.

(Future commits will add the remaining config classes, plus results
and stats classes.)

These classes:

• Are immutable, with builders, but the builders do no validation
  beyond null checks
• Are convertible to and from X-Content, but NOT wire transportable
• Have lenient parsers to maximize compatibility across versions
• Have the same class names, member names and getter/setter names
  as the corresponding classes in X-Pack core to ease migration
  for transport client users
• Don't reproduce all the methods that do calculations or
  transformations that the the corresponding classes in X-Pack core
  have

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

This is labelled >non-issue as it's useless on its own. It's groundwork for future changes that will make ML endpoints available in the HLRC.

[davidkyle]

Looks good, I focused on the comments skimming the code assuming that it is a fairly straight forward port of the originals.

[davidkyle]

Hmm I wonder how old the original comment is.

We should reference the docs here detector config and function reference

"Defines the fields and functions used in the analysis. A combination of field_name, by_field_name and over_field_name can be used depending on the specific function chosen. For more information see configuring detectors and detector functions"

[davidkyle]

LGTM