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/traits.rs

@@ -467,6 +467,8 @@ pub trait CommonUser: Send + Sync {
     async fn db_metrics(&self) -> oj_rc_database::DatabaseMetrics;
     async fn db_counters(&self) -> Vec<(&'static str, i64)>;
     async fn currency(&self, ty: CurrencyType, op: CurrencyOp) -> Result<u64, polariton_server::operations::SimpleOpError>;
+    async fn fedi_get(&self) -> super::Federation;
+    async fn fedi_set(&self, fedi: super::Federation) -> bool;
 }
 
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]