feat!: Decode returns opaque event objects when a formatter is not available

[dazuma]

• Added a class for opaque event data
• Added an option to tell HttpBinding#decode_rack_env to return an opaque object if the format isn't recognized
• Deprecated the HttpContentError class and instead introduced UnsupportedFormatError, to keep error classes binding-agnostic.
• Introduced FormatSyntaxError to report parsing errors during decoding. These were previously reported as raw JSON errors. (The JSON errors are still set as the cause.)
• Introduced HttpBinding#encode_event as a single entrypoint for encoding. Deprecated encode_structured_content, encode_batched_content, and encode_binary_content.

Signed-off-by: Daniel Azuma [email protected]