Generate an enum listing all generated operations, add FilterByOperation plugin

[hlbarber]

As stated in #1837 (comment), a type safe alternative to matching against &'static str would be to generate an enum whose variants are a list of the operations and then match against that.

We should still attach this enum to the OperationShape as an associated type.

With this we can then develop a FilterByOperation plugin.

• Write a design proposal
• Implement the accepted solution