Provide a mechanism for setting the version Option

The `SnapshotBuilder` is a reasonable improvement IMHO, but the fluent
style because challenging when the caller has an `Option<u64>` as well.

I have been fond of the [AWS SDK fluent
builders](https://docs.rs/aws-credential-types/1.2.6/aws_credential_types/struct.CredentialsBuilder.html#method.set_account_id)
which provide a `account_id(id)` and `set_account_id(Option<id>)` style
builders which give callers flexibility.

Instead of:

```rust
let mut builder = Snapshot::builder(url);
builder  = match version.as_ref() {
  Some(version) => builder.at_version(version),
  None => builder,
};
let snapshot = builder.build(engine.as_ref();
```

Callers can instead:

```rust
let snapshot =
Snapshot::builder(url).set_version(version).build(engine.as_ref());
```

Signed-off-by: R. Tyler Croy <[email protected]>

Author: rtyler

kernel/src/snapshot/builder.rs

@@ -43,6 +43,13 @@ impl SnapshotBuilder {
         self
     }
 
+
+    /// Set the `version` [Option] explicitly.
+    pub fn set_version(mut self, version: Option<Version>) -> Self {
+        self.version = version;
+        self
+    }
+
     /// Create a new [`Snapshot`] instance.
     ///
     /// # Parameters