add base64 as explicit dependency

[HoneyryderChuck]

closes #2984

was made into a bundled gem in ruby 3.4

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]

I was looking at doing this soon. I would like to add it to all gems where it is used.

[HoneyryderChuck]

@mullermp feel free to close it if you are already working on this. my change was mostly "drive by" to fix my local issue, but you're right that all aws-sdk-* gems should declare new bundled gems individually.

[mullermp]

I haven't started. We can use this PR. I'll look today.

[mullermp]

I just took a look. I see there are some individual gems that need base64 but I think it's entirely reasonable to get those through cores. What I was looking at was non aws-sdk gems such as aws-sigv4, aws-eventstreams, etc, and it appears base64 is used during tests, which does not need to be included. So, I think as a whole, this change is just fine.