[EG] decode method

[rakshith91]

• make them static
• make sure it accepts a list as payload
• Add data tests that use the u”” syntax and the b”” syntax
• data on event should be symmetric - Base64 auto-encode/decode on both send/decode.
• add _to_json
• add sys event mappings to samples

[rakshith91]

Closing in favor of #16433