The CloudEventMessageBuilder does not respect the cloudevents specification

[vidourem]

The CloudEventMessageBuilder instantiates the time header with an OffsetTime and this does not allow the date to be transmitted. I think you have to use an OffsetDateTime.

[vidourem]

Hi @olegz,

Thank you for the commit and your contribution on the project
I notice too late that the org.springframework.cloud.function.cloudevent.CloudEventMessageUtils class also uses an OffsetTime instead of OffsetDateTime.

For my initial remark on the date format, it's more about the serialization of dates in general. spring-cloud-function is not affected by this remark.