Provide a base64 primitive

[shs96c]

When calculating things like integrity hashes (used by recent Bazel releases), it's helpful to be able to base64 encode and decode values. Starlark does not offer a mechanism to do this natively, though it may be possible to write one in pure starlark. Having a native implementation would be significantly faster (as we saw with JSON processing)

[shs96c]

One such pure Starlark implementation is here: https://github.com/aspect-build/bazel-lib/blob/main/lib/private/base64.bzl

[brandjon]

It seems like this would be supported by a string <-> bytes encoding/decoding API. So I think this is blocked on finalizing bytes support in the spec (the list of methods, which are currently TODOs).

One relevant question is whether base64 would be listed in the spec as a required supported encoding for any implementation.