This commit defines the message format used to notify external parties of
activity within a registry instance. The event includes information about which
action was taken on which registry object, including what user created the
action and which instance generated the event.
Message instances can be sent throughout an application or transmitted
externally. An envelope format along with a custom media type is defined along
with tests to detect changes to the wire format.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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