feat: change CloudEvent time property from string to object (#66)

[TonyVlcek]

Closes #66

Since Z and +00:00 are aliases in RFC 3339 and PHP happens to output +00:00 when the P (and thus the RFC3339_EXTENDED) format character is used, I chose to update the tests accordingly so that the DateTimeInterface:RFC3339_EXTENDED can be used. However, if you'd prefer for the tests to stay the same and for the Z character to be printed instead of +00:00, it will be a simple tweak.