Caveat update

[rekmarks]

• Export CaveatTypes enum
• Simplify and refactor caveat validation
  • Previously, we did not check that the caveat type was supported. Now we do. A test case has been added.
• Rename requireParams caveat to requireParamsIsSubset
  • To better reflect the action of the caveat, which is to reject the request if the params aren't a subset of the caveat value.

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P.S. My apologies to posterity for the awful commit name.