- Lower priority is used for triggers so other plugins can add headers before the XML is
generated.
- Added `headers` to `MessageDataExtra` type.
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- SHIM is now supported and put in `msg_data["extra"]["headers"]`.
- `Keywords` are converted from and to list of string in `msg_data["extra"]["keywords"]`
field (if present in headers on message sending, values are merged).
- Python minimal version upgraded to 3.11 due to use of `StrEnum`.
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