Backends can only return a single public/private key from calls to Backend::find_(private|public)_key()

FindObject returns a list of objects, see https://github.com/google/native-pkcs11/blob/main/native-pkcs11-traits/src/lib.rs#L191. This is done in 2 stages, firstly call the backend to populate the object_store, then apply a filter to the object store to return requested objects.

However, the Backend trait only returns a single object from the find_(private|public)_key() methods.
https://github.com/google/native-pkcs11/blob/main/native-pkcs11-traits/src/lib.rs#L191

```
pub trait Backend: Send + Sync {
fn name(&self) -> String;
fn find_all_certificates(&self) -> Result>>;
fn find_private_key(&self, query: KeySearchOptions) -> Result>>;
fn find_public_key(&self, query: KeySearchOptions) -> Result>>;
fn find_all_private_keys(&self) -> Result>>;
fn find_all_public_keys(&self) -> Result>>;
```

This means that a find for a public or private key will only return a single object.

However, to make this more confusing, if a caller has already requested all keys (via find_all_private_keys() or find_all_public_keys() , then multiple keys may be returned, as these methods populate the object_store with all objects.

A suggested fix is to change the find_private_key() and find_public_key() methods to return a Vec<>, as per the find_all_*() methods.