Support namespace and labels

[carolynvs]

• Namespaces allow us to scope documents to a particular namespace or leave empty to define it in the global namespace.
• Labels let us define key/value pairs on data stored that we can use to query later.
• Add labels to bundles, credential sets, installations, claims, results, and outputs
• Add namespaces to credential sets, installations, claims, results, and outputs.

Bundles already had the concept of namespacing via using the format com.example.mybundle. I did not want to use this format for the other documents and opted to use a separate field to preserve existing behavior. People could have named their other documents with periods not intending for it to be interpreted as namespaces.

In the examples for Credential Sets, I change it to reference how Porter does it because Duffle is archived and won't implement the new fields.

Closes #408

[carolynvs]

@jlegrone Do you think that label sources should be at the spec level? I'm also wondering what sources we would support initially, maybe

• bundle (labels defined by the bundle.json)
• invocation image (labels added to the installation or claim by the executing invocation image)
• user (set by the user manually)

[carolynvs]

I'm going to follow Butcher's advice from https://github.com/cnabio/cnab-spec/issues/410#issuecomment-805345680 and start a new spec for this stuff.