Use upstream cpp-base64 git url

```
set(cpp_base64_GIT_REPOSITORY "https://github.com/CodeFinder2/cpp-base64.git")
```

Ideally improvement to dependency should be contributed upstream and if these contribution are under review and waiting for their integration is not possible, a more permanent fork should be created outside of a user specific account (e.g https://github.com/InsightSoftwareConsortium)

Considering that https://github.com/CodeFinder2/cpp-base64/commit/9144cd53be930b37235ae552a92b5d2aa51e9325 is available in the upstream, I then suggest to use it.

_Originally posted by @jcfr in https://github.com/InsightSoftwareConsortium/itk-wasm/pull/670#discussion_r998600613_