Add consul snapshot decode command

[mkeeler]

Description

Often in debugging Consul, users want to be able to see what is in a snapshot. The snapshot data itself is encoded in a non-human readable format. This PR introduces a consul snapshot decode command. This command reads in a snapshot and outputs a stream of line delimited JSON objects. The expectation is that the user can then post-process the stream with some other tool like jq to find/display some relevant data.

Each object at the top level has the form: {"Type": "<type name>", "Data": <JSON encoded data value>}. The <type name> comes from the string form of the request type. In addition to all the various request types specified in the structs package, a SnapshotHeader type is also used to pass back the snapshot header metadata to the user within the JSON object stream.

For forwards compatibility and for a CE binary operating on an enterprise snapshot, if the request type byte isn't known to the CLI then the <type name> will be Unknown(<type byte integer value>).

Testing & Reproduction steps

• New unit tests including some golden file tests.