Introduce types.Any to be used in place of json.RawMessage

[chungers]

This PR introduces a common type Any that models any user-defined / opaque structures in the input configuration.

Currently when a plugin defines its custom input, a Spec, this code pattern is used:

type Spec struct {
    Plugin string  // the name
    Properties *json.RawMessage // the custom blob
}

To provide better typing and a common abstraction (and not be so tightly coupled with JSON), the new pattern, including the plugin.Name introduced in #362, now look like:

type Spec struct {
    Plugin plugin.Name
    Properties *types.Any
}

• Any has simple utility functions that help avoid boilerplate json.Marshal/Unmarshal code everywhere.
• Provides a common object that can be used everywhere that will make support for additional formats easier in the future (e.g. YAML configs)
• Any is currently typed aliased to json.RawMessage but this can be changed in the future without impacting code.
• Any is similar in concept to the type Any in protobuf3.
• Converted the Group plugin's Spec to use Any.
• Converted the plugin launcher utility to use Any for configuration.

This PR does not change the plugin SPI where the use of json.RawMessage appears in methods like Validate. I'd like to propose that these be converted to use Any by v1.0.

[codecov-io]

Current coverage is 65.38% (diff: 60.65%)

Merging #363 into master will increase coverage by 0.22%

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