[EuiPopover] Allow adjusting buffer for individual window sides

[cchaos]

We currently have an issue where, because we need the popover z-indexes to stay above EuiHeaders, when scrolling the anchor underneath the header, the popover becomes disconnected from the anchor.

We've talked about using the container prop but this won't exactly work correctly since the headers are fixed position. The easiest way then is to be able to pass a larger buffer for the top of the window to account for the fixed headers.

Something like:

  /**
   * Minimum distance between the popover and the bounding container;
   * Pass an array of 4 values to adjust each side differently: `[top, right, bottom, left]`
   * Default is 16
   */
  buffer?: number | number[];

Or we could ask for a specific map, but I figured this would map similarly to CSS padding/margin shorthands.

[cchaos]

Potentially related issues: #3845, #4232