Support for validation across parameters

[sazzer]

Right now in OpenAPI 3.x you have the ability to specify header, path and query parameters and to validate those parameters. Wha you don't have the ability to do is to have validation that spans across them.

For example, I might have a resource that supports two parameters:

• sortBy
• sortOrder

Where sortOrder is mandatory if sortBy is specified, but prohibited otherwise. Right now you can't do this, and instead you are forced to depend on free-text documentation of the fields.

Other things that would be useful here include, but not limited to:

• Restricting the values of one parameter based on the values of other parameters - as opposed to just the presence/absence of them.
• Changing the way parameters are treated based on HTTP headers.
• Mutually exclusive parameters - can have ether a or b but never both.

[handrews]

See also #20 and #24.

[handrews]

I'm going to close this in favor of #100, which got started by someone who did not realize this discussion already existed and now has a much longer set of comments. Please feel free to add to that discussion!