Support `base64` encoding of `bytes` as `str` in `Document`

[blythed]

Add an optional parameter bytes_encoding to Document.encode so that we can send encoded Document instances via REST interfaces.

bytes_encoding can take 2 values: "bytes" and "base64" - i.e. config.BytesEncoding

Tasks

• [x] Add override of CFG.bytes_encoding in:
  • [x] Encoder.encode
  • [x] Encoder.decode
  • [x] Encodable.encode
• [x] Add override of CFG.bytes_encoding in Document.encode and Document.decode
• [x] Unittest checking Document({'img': pil_image(PIL.Image.open(...))}).encode(bytes_encoding=BytesEncoding.base64)