Add a get node function to the DJ client

[shangyian]

Summary

There are a few changes in this PR:

• Adds a get node function (dj.node(<node_name>)) to the client that is agnostic to the node type. This is useful when someone has a list of node names and needs to call the client to retrieve a node (if one exists) for each name.
• Fixes a bug with the metrics returned by the client's cube node retrieval (it was returning the metric node name + the metric column name, but the column name doesn't need to be there).
• Fixes a bug where cube node updates don't work because we were comparing a string with an enum and not its value

Test Plan

• [ ] PR has an associated issue: #
• [ ] make check passes
• [ ] make test shows 100% unit test coverage

Deployment Plan

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