fix(ion-datetime): convert DatetimeData to ISO string

[kensodemann]

Short description of what this resolves:

Modifies the datetime component to emit the ISO string for the value changed when the value changes. The value itself gets set to the picker structure, so that is no good as something to emit.

NOTE: it may at some point be better to rewrite the component to not use this.value as the value returned from the picker, but that would be a much larger and much more error prone rewrite at this point. I would want proper tests in place first.

Changes proposed in this pull request:

• emit the ISO date representation of the value rather than the value itself
• modify the CVA to pick up the emitted value rather than the actual value in the target
• start adding unit tests

Ionic Version: 4.x

Fixes: #15408

[manucorporat]

The PR looks great, but wondering if event.detail.value !== event.target.value could be a problem. The rest of input component maintain a consistency regarding value

[kensodemann]

I believe what I did in PR #15907 to fix this makes a lot more sense, so I am going to close this one.