Add RFC3339 timestamp

[omarismail]

Background

Go has a time format called RFC3339.

RFC3339     = "2006-01-02T15:04:05Z07:00"

PR Description

This PR introduces the ability to add an annotation jsonapi:"attr,next,rfc3339" to a time.Time attribute, and it will parse the time according to the Go RFC3339 time format.

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[omarismail]

@googlebot I signed it!

[quetzyg]

@aren55555 this PR was merged without the proper updates in the response code.

Mainly here, here and here.

[aren55555]

@quetzyg yes I tried this last night and it appeared supplying the annotation resulted in the time still being serialized as a numeric value rather than a string. Reverted it.

[aren55555]

I've corrected this in #204