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/config/traits.rs

@@ -46,6 +46,8 @@ pub trait ConfigProvider<C: Clone> {
     fn garage_slot_limit(&self) -> i32;
     fn team_choosers(&self) -> TeamChoosers;
     fn redacted_json(&self) -> String;
+    /// None when federation is not enabled
+    fn federation(&self) -> Option<Federation>;
 }
 
 pub struct DevMessageProvider<C: Clone> {
@@ -579,3 +581,8 @@ pub struct PlatformConfig {
 pub trait RedactedClone {
     fn redacted_clone(&self) -> Self;
 }
+
+pub struct Federation {
+    pub aliases: std::collections::HashMap<String, String>,
+    pub defederated: Vec<String>,
+}