Move DropTargetRowContainer static styles to CSS class for user customization

[tenpaiyomi]

Description

Updates the DropTargetRowContainer to change the div to have a defined className, allowing for users to modify the CSS styling as desired.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

DropTargetRowContainer defined its CSS style rules inline, making them unable to be changed easily.

What is the new behavior?

Outside of top and height, which are defined from props, the rest of the CSS rules have been moved into react-data-grid-drop-target.css under a class name of rowDropTarget. This allows a user to define a CSS rule and create their own styles. The functionality of everything remains the same.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No