Enable multi tenancy for email link sign in (#7246)

Author: renkelvin

FirebaseAuth/Sources/Backend/RPC/FIREmailLinkSignInRequest.m

@@ -43,6 +43,11 @@
  */
 static NSString *const kPostBodyKey = @"postBody";
 
+/** @var kTenantIDKey
+    @brief The key for the tenant id value in the request.
+ */
+static NSString *const kTenantIDKey = @"tenantId";
+
 @implementation FIREmailLinkSignInRequest
 
 - (instancetype)initWithEmail:(NSString *)email
@@ -66,6 +71,9 @@ - (nullable id)unencodedHTTPRequestBodyWithError:(NSError *_Nullable *_Nullable)
   if (_IDToken) {
     postBody[kIDTokenKey] = _IDToken;
   }
+  if (self.tenantID) {
+    postBody[kTenantIDKey] = self.tenantID;
+  }
   return [postBody copy];
 }