[CloudEvent] allow custom encoding

[rakshith91]

allow data other than base64 encoding and no encoding

• check if the swagger supports it

Currently, when sending a cloud event, we encode the bytes to base_64 and send it as data_base64
if it's a string (python 3), we simply send it without any encoding as data.

The users should be allowed to send data with a custom encoding. For example, If a user wants to do base8 encoding for the data, they should be allowed to do so. (perhaps send it as data?).

Open for design discussions on how to implement it

[lmazuel]

Closing, bytes are sent with data_base64, and string as data, that's enough