[Test-Proxy] Custom matcher for body keys only

[mccoyp]

Key Vault tests currently use a custom json_attribute_matcher with vcrpy as a custom request matcher. This checks that the keys in request bodies match, without enforcing an ordering and without comparing body values.

It would be great if we could support this kind of matching with the test proxy, as it probably would have uses outside of KV for tests that have consistently-structured bodies with possibly (expectedly) inconsistent values.

@scbedd

[scbedd]

@pshao25 has just requested this in general for application/json. I'm leaning towards that being a better behavior than having a custom matcher, though a custom matcher will be a great way to integrate it.

Frankly I think it's probably a reasonable request. I don't think that it would have negative impact to the existing tests. If anything this lowers the restriction on matching.

Modify here