feat(aws-elasticache): add ServerlessCache, User, and UserGroup L2 Constructs

[mazyu36]

Issue # (if applicable)

Closes #55.

Reason for this change

To support Serveless Cache L2 Construct.

Description of changes

Ad the following Constructs

• ServerlessCache
• User
• UserGroup

Description of how you validated changes

Add unit tests and integ test.

Checklist

• My code adheres to the CONTRIBUTING GUIDE

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

[mazyu36]

There are several validations and configurations that are not documented in the CFn documentation but have been added based on observations from the management console.

I've left these as review comments, so please resolve if there are no problems.

[go-to-k]

More general would be better.

[go-to-k]

Did we get all the threads resolved? (let me know if I missed any)

I think I've seen everything else, nothing appears to be wrong (which is so great).
I'll merge this PR if you are OK, but I'd like you to give me a final check just to be sure.

[mazyu36]

Thank you.
I have addressed all the comments, but I will double-check everything tomorrow and get back to you.

[mazyu36]

@go-to-k
I fixed several typos in the README.
I also improved the explanation and implementation samples for the original default user that is automatically created in AWS.
Additionally, I added an integration test for the original default user.

Please let me know if this looks good to you.

[go-to-k]

Looks good. May we merge this PR?

[mazyu36]

Thank you. I would appreciate it if you could proceed with the merge.