feat: add S3 import functionality which is supported for MySQL instances

[bryantbiggs]

Description

• add S3 import functionality which is supported for MySQL instances
• add new example for demonstrating and verifying S3 import functionality

this closely mirrors what was implemented at terraform-aws-modules/terraform-aws-rds#289

Closes #126

Motivation and Context

• adds functionality to import Percona Xtrabackup from S3

Breaking Changes

• No

How Has This Been Tested?

• Tested using examples/s3_import module that was added

[antonbabenko]

Thanks @bryantbiggs !

The only thing is that I am not sure we really need 3,4MB zip in this repo which everyone should download now even if they don't use this feature. If people will complain about it we probably need to reduce this file size to the minimum required for example.

v3.7.0 has been just released.

[bryantbiggs]

ya, I'm not a MySQL expert so I wasn't sure if there were things that could be trimmed from the database dump

[github-actions]

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