Allow custom encoders to validate compression options

[kanongil]

While implementing support for the new route level compression option in a custom encoder registered using server.encoder(), I realized that the current implementation either requires me to validate the compression options on each request, or trust that they are correct.

Since neither option is desirable, I created this PR to extend the API with an optional schema to allow the validation logic to be applied only once, when the route is created.

Let me know what you think?

Note that this only validates at the route level, and not at the connection level, since the connection already exits when adding the encoder.

[hueniverse]

This still fails to validate routes added before the encoder/decoder is added. I need to think about other options.

[hueniverse]

I like the idea but this is not the right solution.

[lock]

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