membership: api group membership endpoint (#104)

* added api group handler

* func name fixes

* changed json encoding
diff --git a/core/auth/memberships/go.mod b/core/auth/memberships/go.mod
index 609edd8..2615d74 100644
--- a/core/auth/memberships/go.mod
+++ b/core/auth/memberships/go.mod
@@ -5,6 +5,7 @@
 require github.com/ncruces/go-sqlite3 v0.12.2
 
 require (
+	github.com/gorilla/mux v1.8.1 // indirect
 	github.com/ncruces/julianday v1.0.0 // indirect
 	github.com/tetratelabs/wazero v1.6.0 // indirect
 	golang.org/x/sys v0.18.0 // indirect