bug: relay publish fails with 400 Bad Request when message contains an unknown field

[fbarbu15]

Problem

Similar to #2436 but in this case the field should be just ignored

To reproduce

1. Use a payload like {"payload": "RmlsdGVyIHdvcmtzISE=", "contentTopic": "/test/1/waku-filter/proto", "timestamp": 1707989890656837632, "extra": "extra"}

Actual behavior

400 Bad Request with Invalid content body, could not decode. Unable to deserialize data

Expected behavior

Should work and the extra field should be ignored