bug: ephemeral field is ignored and not returned when retrieving messages even if the message contained this field

[fbarbu15]

To Reproduce

1. Publish a message that contains the ephemeral field
2. retrieve that message using relay/v1/messages or filter/v2/messages

Expected behavior
Would expected to see the field in the get response. For optional fields like meta or version it works fine, they are present when retrieved.

Actual behavior
The message is retrieved without the ephemeral field.

go-waku version/commit hash

latest

Related to waku-org/nwaku#2436