MBD: edge specifiers

[adamchalmers]

Based on #1239 but with a few improvements:

• The JSON deserializer checks that at least one of the variants was set, making impossible states unrepresentable
• The JSON deserializer chooses which one to use if both were set, reducing engine code
• The Rust builder API forces users to set one of the variants, making impossible states unrepresentable

[adamchalmers]

Closing because open-api lint complains about the single parameter accepting multiple types (either UUID or edge specifier) which is really annoying because I like this API but it's got a good point, it's hard for client generators to deal with.