Add RBS signatures for customizations of s3

[ksss]

I wrote the RBS for the customizations code.

When performing type checks, emphasis is placed on correcting undefined method errors, and the types of arguments are not specified in detail.

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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.

1. To make sure we include your contribution in the release notes, please make sure to add description entry for your changes in the "unreleased changes" section of the CHANGELOG.md file (at corresponding gem). For the description entry, please make sure it lives in one line and starts with Feature or Issue in the correct format.

2. For generated code changes, please checkout below instructions first:
   https://github.com/aws/aws-sdk-ruby/blob/version-3/CONTRIBUTING.md

Thank you for your contribution!

[mullermp]

Thanks for adding these! At a glance these look ok. I can take a deeper look tomorrow night / Monday and get it merged then. Approving CI for you now.

[mullermp]

Thank you! If you can address that one change (or provide input otherwise), I'd be happy to merge this.

[ksss]

Thank you for reviewing. I resolved!

[alextwoods]

Thanks for adding these!