feat(rds): Support SQL Server for RDS proxy (#25102)

Closes #22164

[As stated in the issue the iam auth has a third option with the support for sql server.](#22164 (comment)) I did not add it in this pull request, because replacing the boolean property iamAuth with a string property would lead to breaking changes.
We should open up another issue for this and fix this separately.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: daschaa

packages/@aws-cdk-testing/framework-integ/test/aws-rds/test/integ.proxy-sql-server.js.snapshot/aws-cdk-rds-proxy-sql-server.assets.json

@@ -0,0 +1,19 @@
+{
+  "version": "31.0.0",
+  "files": {
+    "b4a5559af4ad6c38f2ee9f74b53b1b86efd673d82a598a085654e2c921d8edda": {
+      "source": {
+        "path": "aws-cdk-rds-proxy-sql-server.template.json",
+        "packaging": "file"
+      },
+      "destinations": {
+        "current_account-current_region": {
+          "bucketName": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}",
+          "objectKey": "b4a5559af4ad6c38f2ee9f74b53b1b86efd673d82a598a085654e2c921d8edda.json",
+          "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}"
+        }
+      }
+    }
+  },
+  "dockerImages": {}
+}