registry-experimental: "decode" command for decoding known experimental protobuf types

[timburks]

registry-experimental decode TYPE would read binary data from standard input and attempt to unmarshal it as an encoded proto of the specified type, and then write the result as JSON (by default) or YAML. TYPE could be a fully-resolved type or, when unambiguous, a known message name. For example, the following invocations might get and decode messages of type google.cloud.apigeeregistry.applications.v1alpha1.Index:

registry get artifacts/index --raw | registry-experimental decode google.cloud.apigeeregistry.applications.v1alpha1.Index

registry get artifacts/index --raw | registry-experimental decode Index

This would allow us to move all of the v1alpha1 protos out of the main registry repo and into this one.

registry-experimental decode --list-types might be useful for printing a list of known types.