[Vertex AI] Replace "enum" computed properties with static let

[andrewheard]

Replaced computed properties with stored type properties (static let) in the structs conforming to ProtoEnum. Minor optimization - no change to the public API surface.

Interesting bit from the Swift documentation:

Stored type properties are lazily initialized on their first access. They’re guaranteed to be initialized only once, even when accessed by multiple threads simultaneously, and they don’t need to be marked with the lazy modifier.

#no-changelog