feat(dropdowns): adds support for fixed positioning strategy

What does this MR do?

feat(dropdowns): adds support for fixed positioning strategy

This adds support for the fixed positioning strategy, which can be useful when a dropdown is contained within a short element that might cause it to be cut off. Applying this positioning strategy would let the dropdown position itself above the container, ignorning its size constraints.

This is done primarily to migrate gitlab!120393 (merged) to Floating UI. In that MR, the popper-options prop was used to set a custom positioning strategy, which was not taken into account when we upgraded to Floating UI. This restores the ability to set the strategy via a new positioningStrategy.

Screenshots or screen recordings

Before (using Floating UI without this fix)	After

Integration merge requests

• GitLab: gitlab!121070 (merged)
• [-] CustomersDot: mr_url
• [-] Status Page: mr_url

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Accessibility

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