Add ability to have constants

[rspremulli-sf]

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Use case:

I have a discriminated type, and certain properties, such as the discriminator value itself, may only accept a single valid value. There is a need to pass the value to identify the type, but there is only one allowed value. Ideally, we could have a constant facet that simply says, "this is the value you must pass"

Current workaround:

required value with an enum that has only one value in it.