Switch to `base64` crate #1294

This removes our implementation of base64 encoding/decoding from `aws-smithy-types`, switching to the `base64` Rust crate. Our implementation was written at a time when we wanted to keep dependencies at an absolute minimum. However, a performant base64 implementation is now needed for the server. Our implementation was also accepting invalid base64 (see #1277). Fixes #1277.