Log Based Registry: Operator Entries

Associated with proposal #25

There are use cases where registry operators may need to add entries to the log. For example,
* to add annotations that the maintainers can not opt-out of
* or to transfer ownership of the package in the event of inactivity, abuse, or admin key loss.

Non-operator keys are always added using the Assign Role entry, but there is the option to maintain the operator key list either within or outside the log.

For example, the operator could be required to post operator keys to a transparency log with an issue and expiry timestamp. Validators would then collect this information use it to validate operator entries using their timestamps. On the other hand, operators could post their keys in the log so that it is tracked within the existing log system.