Add collation for an index via @CompoundIndex and @Index annotations.

[StefanTirea]

Closes #3002, closes #4130

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Hi,
I recently wanted to use Collation with Spring Data and saw there is no support to implement it only with annotations for a specific field.
A use case would be to have an unique index which is case insensitive.
My PR adds support to use it with annotations (currently it is only possible programmatically).
I hope the implementation did not miss an obvious hurdle for this feature.

Still missing in this PR:

• Proper documentation in @Index & @CompoundIndex (see TODO)

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[christophstrobl]

Thank you @StefanTirea! Merged to main.