chore: implement databased backend for custom roles (#13295)

Includes db schema and dbauthz layer for upserting custom roles. Unit test in `customroles_test.go` verify against escalating permissions through this feature.

coderd/rbac/rolestore/rolestore.go

@@ -0,0 +1,37 @@
+package rolestore
+
+import (
+	"encoding/json"
+
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/rbac"
+)
+
+func ConvertDBRole(dbRole database.CustomRole) (rbac.Role, error) {
+	role := rbac.Role{
+		Name:        dbRole.Name,
+		DisplayName: dbRole.DisplayName,
+		Site:        nil,
+		Org:         nil,
+		User:        nil,
+	}
+
+	err := json.Unmarshal(dbRole.SitePermissions, &role.Site)
+	if err != nil {
+		return role, xerrors.Errorf("unmarshal site permissions: %w", err)
+	}
+
+	err = json.Unmarshal(dbRole.OrgPermissions, &role.Org)
+	if err != nil {
+		return role, xerrors.Errorf("unmarshal org permissions: %w", err)
+	}
+
+	err = json.Unmarshal(dbRole.UserPermissions, &role.User)
+	if err != nil {
+		return role, xerrors.Errorf("unmarshal user permissions: %w", err)
+	}
+
+	return role, nil
+}