[BEAM-121] Additional APIs for registering DisplayData

[swegner]

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[swegner]

R: @bjchambers

This PR is based on feeback from #123, #124, #125, #126. I will update those PRs to consume these APIs once merged.

[bjchambers]

namespaceOf(...)? classToNamespace? (just thinking out loud, feel free to ignore if you don't think they're improvements).

[bjchambers]

LGTM

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Only thing that occurred to me is that we could reduce the number of methods on the builder by putting more logic into the Type enum. Let's stick with this for now since the other PRs are based on the existing API, but it may be worth thinking about something like:

Type.STRING.entry("key", "value")
Type.STRING.entryIfNotNull("key", value)
Type.STRING.entryIfNotDefault("key", value, defaultValue)

Then the builder API just needs a method to add an entry if its not null:

builder.add(Type.STRING.entryIfNotDefault("key", value, "default"))

This is also nice when including the inference:

builder.add(Type.infer(value).entry("key", value))

[swegner]

Backported via GoogleCloudPlatform/DataflowJavaSDK#216