Include base64 in folly dependency, unbreak CI (#45417)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45417

Add sources for `folly::detail::base64` to our existing `folly` dependency for Android and iOS OSS build toolchains, now a (tiny) dependency of the JS debugger since D54309633, and already part of Folly's Buck sources.

Changelog: [Internal]

Reviewed By: cipolleschi, blakef

Differential Revision: D59685218

fbshipit-source-id: bac33402927f310bf867d2c47b4ebbb9276cf545