Replace use of Skia's Base64 Encoding/Decoding logic with a copy of the equivalent code (#46543)

Skia would like to remove SkBase64.h from its public API. This ports the
same functionality into Flutter's codebase with tests.

The implementation was copied from
[Skia](https://github.com/google/skia/blob/387853af198f03b4634239fb92cac5d49fe37100/src/utils/SkBase64.cpp)
then modified to match Flutter's style and have readable tests.

In a follow-up PR, I would like to add a function to pre-flight the
calculation needed to figure out how many
bytes are needed to be allocated, to avoid the clunky double API
call (see the TODOs).

I chose to put the code in `//shell/common` at the suggestion of Flutter
devs, but it needs to be in its own source_set because it is used in a
few other places (for now) and we want to avoid dependency cycles.

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- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

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