chore: add query to fetch top level idp claim fields (#15525)

Adds an api endpoint to grab all available sync field options for IDP
sync. This is for autocomplete on idp sync forms. This is required for
organization admins to have some insight into the claim fields available
when configuring group/role sync.

coderd/database/dbmem/dbmem.go

@@ -8409,6 +8409,35 @@ func (q *FakeQuerier) ListWorkspaceAgentPortShares(_ context.Context, workspaceI
 	return shares, nil
 }
 
+func (q *FakeQuerier) OIDCClaimFields(_ context.Context, organizationID uuid.UUID) ([]string, error) {
+	orgMembers := q.getOrganizationMemberNoLock(organizationID)
+
+	var fields []string
+	for _, link := range q.userLinks {
+		if organizationID != uuid.Nil {
+			inOrg := slices.ContainsFunc(orgMembers, func(organizationMember database.OrganizationMember) bool {
+				return organizationMember.UserID == link.UserID
+			})
+			if !inOrg {
+				continue
+			}
+		}
+
+		if link.LoginType != database.LoginTypeOIDC {
+			continue
+		}
+
+		for k := range link.Claims.IDTokenClaims {
+			fields = append(fields, k)
+		}
+		for k := range link.Claims.UserInfoClaims {
+			fields = append(fields, k)
+		}
+	}
+
+	return slice.Unique(fields), nil
+}
+
 func (q *FakeQuerier) OrganizationMembers(_ context.Context, arg database.OrganizationMembersParams) ([]database.OrganizationMembersRow, error) {
 	if err := validateDatabaseType(arg); err != nil {
 		return []database.OrganizationMembersRow{}, err