Remove our `base64` implementation in favour of `base64-simd` (#1938)

* Replace our base64 implementation with one based on `base64-simd`

* Bump Rust MSRV to 1.62.1

* Unmark base64-related tests as failing

* Use `usize` for `base64::encoded_length`

* Run Rust 1.62.1 on CI

* Don't use external `base64_simd::Error` type in public interface

* Add initial criterion bench setup for base64

* Compare `base64_simd::encode` vs Smithy's `encode`

* Use string length for `BenchmarkId`

* Compare `base64_simd::decode` vs Smithy's `decode`

* Compare `base64_simd::encoded_length` vs Smithy's `encoded_length`