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e.g. basic usage examples

Pull request open: typelevel/steward#29

This needs some brainstorming on what the API should look like and how to balance ergonomics with type safety.

TODO: add some strawman ideas

Ideas from TL submission:

• expose more functionality from the Java SDK
• add support for server-side ScalaJS, backed by the Javascript server SDK
• add support for client-side ScalaJS
• introduce the concept of a codec for reading feature flags to arbitrary types
• introduce a thicker representation of "feature flag", encapsulating feature key, default value and codec
• add a Scala-idiomatic API for constructing evaluation contexts

When using the default configuration it should be possible to create the client by calling LaunchDarklyClient.resource(sdkKey), similar to how there is a one-argument constructor on the underlying LDClient.

This is largely a quality-of-life suggestion to make encoder definitions less painful, especially when they have only one or two fields.

Add a def simple() method on the ContextEncoder companion object, such as:

def simple[Ctx](_kind: String)(_key: Ctx => String): ContextEncoder[Ctx] = new ContextEncoder[Ctx] {
  ...
}

Alongside this define methods to allow additional attributes, e.g.

def withStringAttribute(attributeName: String)(f: Ctx => String): ContextEncoder[Ctx] = ...

def withBooleanAttribute(attributeName: String)(f: Ctx => Boolean): ContextEncoder[Ctx] = ...

These can then be used together, for example:

implicit val anEncoder: ContextEncoder[Type] =
  ContextEncoder
    .simple[Type]("kind")(key = type.someField)
    .withStringAttribute("otherKind")(_.someOtherField)