[DATATYPE] Migrate `CFG.bytes_encoding` to databackend

[blythed]

This prevents the need to reconfigure on a case-by-case basis.

class Databackend:
    bytes_encoding = 'bytes'

class IbisDatabackend:
    def __init__(self):
        if self.uri.startswith('snowflake'):
            self.bytes_encoding = 'str'
        ...

TODO: only do conversion between bytes and str if encodable == 'encodable'.