[feature] Implement /api/v1/instance/peers endpoint (#660)

* add missing license headers

* start adding instance peers get

* rename domainblock.go

* embed domain in domainblock so it can be reused

* update swagger docs

* add test instances to db

* update tests

* add/update instancepeersget

* update domain model

* add getinstancepeers to db

* instance-expose-peers, instance-expose-suspended

* add auth checks for both current filters

* attach endpoint to router

* include public comment

* obfuscate domain if required

* go mod tidy

* update swagger docs

* remove unnecessary comment

* return 'flat' peerlist if no query params provided

docs/configuration/instance.md

@@ -0,0 +1,26 @@
+# Instance
+
+## Settings
+
+```yaml
+###########################
+##### INSTANCE CONFIG #####
+###########################
+
+# Config pertaining to instance federation settings, pages to hide/expose, etc.
+
+# Bool. Allow unauthenticated users to make queries to /api/v1/instance/peers?filter=open in order
+# to see a list of instances that this instance 'peers' with. Even if set to 'false', then authenticated
+# users (members of the instance) will still be able to query the endpoint.
+# Options: [true, false]
+# Default: false
+instance-expose-peers: false
+
+# Bool. Allow unauthenticated users to make queries to /api/v1/instance/peers?filter=suspended in order
+# to see a list of instances that this instance blocks/suspends. This will also allow unauthenticated
+# users to see the list through the web UI. Even if set to 'false', then authenticated users (members
+# of the instance) will still be able to query the endpoint.
+# Options: [true, false]
+# Default: false
+instance-expose-suspended: false
+```