Added KMS encryption for Aurora DB secrets (#935)

### Feature or Bugfix
<!-- please choose -->
- Bugfix

### Detail
Added KMS key for encrypting Aurora DB secrets

### Relates
#880

### Security
Please answer the questions below briefly where applicable, or write
`N/A`. Based on
[OWASP 10](https://owasp.org/Top10/en/).

- Does this PR introduce or modify any input fields or queries - this
includes
fetching data from storage outside the application (e.g. a database, an
S3 bucket)? No
  - Is the input sanitized? N/A
- What precautions are you taking before deserializing the data you
consume? N/A
  - Is injection prevented by parametrizing queries? N/A
  - Have you ensured no `eval` or similar functions are used?N/A
- Does this PR introduce any functionality or component that requires
authorization? N/A
- How have you ensured it respects the existing AuthN/AuthZ mechanisms?
N/A
  - Are you logging failed auth attempts?N/A
- Are you using or adding any cryptographic features? N/A
  - Do you use a standard proven implementations? N/A
- Are the used keys controlled by the customer? Where are they stored?
N/A
- Are you introducing any new policies/roles/users? N/A
  - Have you used the least-privilege principle? How? N/A


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

---------

Co-authored-by: Mourya Darivemula <[email protected]>
Co-authored-by: dlpzx <[email protected]>

deploy/stacks/aurora.py

@@ -30,9 +30,7 @@ def __init__(
         super().__init__(scope, id, **kwargs)
 
         # if exclude_characters property is set make sure that the pwd regex in DbConfig is changed accordingly
-        db_credentials = rds.DatabaseSecret(
-            self, f'{resource_prefix}-{envname}-aurora-db', username='dtaadmin'
-        )
+
 
         db_subnet_group = rds.SubnetGroup(
             self,
@@ -65,6 +63,10 @@ def __init__(
             alias=f'{resource_prefix}-{envname}-aurora',
             enable_key_rotation=True,
         )
+
+        db_credentials = rds.DatabaseSecret(
+            self, f'{resource_prefix}-{envname}-aurora-db', username='dtaadmin', encryption_key=key
+        )
 
         database = rds.ServerlessCluster(
             self,