feat: add vm.update() method

[stephenplusplus]

Fixes #477

We currently have a "setMetadata()" method on our VM class. Unlike our other methods by the same name, the VM's will not update the remote resource as a whole-- instead, it only updates the metadata object within the resource. In other words, a VM's structure looks like:

{
  "name": "...",
  "deletionProtection": false,
  "metadata": {
    "myCustomProperty": true
  }
}

vm.setMetadata() will only change the "metadata" object. This PR adds vm.update(), which has control of the whole thing.

Things to note:

• vm.getMetadata() returns the whole record, not just the embedded "metadata" object.
• vm.update() is not atomic. It pulls the latest metadata, merges with the new, then sends the update in another call.

[memyselfandm]

This is exactly what I was looking for, thanks for the quick add.

[codecov]

Codecov Report

Merging #478 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #478   +/-   ##
=======================================
  Coverage   99.60%   99.61%           
=======================================
  Files          22       22           
  Lines       11781    11840   +59     
=======================================
+ Hits        11735    11794   +59     
  Misses         46       46           

Impacted Files	Coverage Δ
src/vm.js	99.56% <100.00%> (+0.02%)	⬆️

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