Add server-wide and user-only federation controls (#133)

### Description

Completes #122

### Please confirm

- [x] I am the legal owner or represent the legal owner of all work submitted (including LLM-generated code, if any)
- [x] I consent to my changes being added to this FOSS project
- [x] I have confirmed that this does not add new errors or warnings with `utils/clippy.sh`
- [ ] This PR used LLMs to generate some or all of the code changes

Reviewed-on: https://git.ngram.ca/OpenJam/rc-servers/pulls/133

rc_core/src/persist/user/federation.rs

@@ -0,0 +1,16 @@
+use serde::{Serialize, Deserialize};
+
+#[derive(Serialize, Deserialize, Clone, Debug)]
+pub struct Federation {
+    pub enabled: bool,
+    pub defederated: Vec<String>,
+}
+
+impl std::default::Default for Federation {
+    fn default() -> Self {
+        Self {
+            enabled: true,
+            defederated: Vec::default(),
+        }
+    }
+}