Add Base45 support

[garydgregory]

Add Base45 support.

Before you push a pull request, review this list:

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create the first internal version, then edited away. Copilot driving Claude Sonet 4.6 Medium, Googl;e Gemini website, ChatGPT website.
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
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