Add a helper function that uses base64::write::EncoderWriter

[Xuanwo]

          We can add a helper function that uses [base64::write::EncoderWriter](https://docs.rs/base64/0.22.0/base64/write/struct.EncoderWriter.html). But I believe it's out of the scope of current PR, maybe worth a new one.

Originally posted by @Xuanwo in #4477 (comment)

BASE64_STANDARD.encode(body) requires the whole slices which adds an extra copy for Buffer. We can use base64::write::EncoderWriter to allow build base64 from Buffer.

[Xuanwo]

Ah, EncoderWriter is not zero cost compared to encode. Maybe it's better to use Buffer::to_bytes() as is. Closing.