Implement explicit operation sunsetting

[cmars]

Implement the explicit operation sunsetting required by the solution proposed in #99.

Proposal here is to introduce two extensions on operations in versioned specs:

• One that marks the operation as deprecated. This is necessary when an operation needs to be sunset without being superseded by one in a newer version.
• One that indicates when the operation was deprecated. A linter rule (Optic CI) will need to assert that this cannot be back- or post-dated, it must match the date it lands.

[cmars]

I think this can be done in a single extension on the operation, x-snyk-deprecated-on, which takes a date of the form YYYY-mm-dd.

Adding x-snyk-deprecated-on to an operation should cause requests to this endpoint to be regarded as deprecated, as if it had been followed by a newer release on the date given. Sunset calculation is determined according to this date and the stability of the operation being deprecated.

A Sweater Comb rule will also be needed to prevent shenanigans, such as back-dating a deprecation.