aws-fargate-opensearch Set Cognito Password Policy request

Provide props that user can set cognito user pool's password policy that minimally specify a password length of at least 8 characters, as well as requiring uppercase, numeric, and special characters.

### Use Case

In building prototyping, there's a cdk-nag that The Cognito user pool does not have a password policy that minimally specify a password length of at least 8 characters, as well as requiring uppercase, numeric, and special characters.
I'd like to set password policy to resolve this error.

### Proposed Solution

### Other

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by locating the aws-fargate-opensearch construct and its Cognito user pool definition, then inspect any existing props and tests for that construct. The work is done when callers can configure the requested password policy and tests verify the minimum length and character requirements.

Written by the indexing model from the issue text.