build: include zeroize dependency in obfuscate crate

So that we can implement `Obfuscate` for `Zeroize<T>` where `T:
Obfuscate`.

Author: SimonThormeyer

Cargo.lock

@@ -12,6 +12,7 @@ openmls = { workspace = true, optional = true }
 openmls_basic_credential = { workspace = true, optional = true }
 rand.workspace = true
 sha2.workspace = true
+zeroize.workspace = true
 
 [lints]
 workspace = true