[uniffi] Enum variants cannot match existing types

[Martin Geisler]

Problem:

Enum variants cannot have names that collide with other types: UniFFI will generate incorrect Kotlin code in that case. See mozilla/uniffi-rs#1853.

Solution:

We should rename our enum variants as needed until this is fixed upstream. It affects us in two cases at the moment:

• mls-rs/mls-rs-uniffi/src/lib.rs

  Lines 178 to 182 in de125ef

   /// A proposal was received. 
   Proposal { 
       sender: Arc<SigningIdentity>, 
       proposal: Arc<Proposal>, 
   }, 

• mls-rs/mls-rs-uniffi/src/lib.rs

  Lines 189 to 190 in de125ef

   /// Validated key package. 
   KeyPackage { key_package: Arc<KeyPackage> }, 

In both cases, the generated code will recursively refer back to the enum variant instead of the top-level type.

We should ideally fix this upstream, but we'll probably just rename the variants for now.