Initialize should be composed of Resets + StatePreparation

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What is the expected enhancement?

In #6236, we discussed the possibility of a StatePreparation class with no resets and a separate Initialize that uses resets + state-preparation. There is a related issue in #1896.

I raised the possibility to remove the resets in #6236 because an alternative to go from a state | a> to a state | b> is to apply the inverse of initialize |a> followed by initialize |b>. Another example of initialization without resets is a controlled initialization as in arXiv:1703.10793. In these cases, a gate StatePreparation should be used instead of an instruction Initialize with resets. The discussion in #1896 does not address these cases.