[UI v2] Variables table - part 1

[desertaxle]

Adds a variables table to the v2 UI. Not all features are included in the table right now. The focus of this PR is display and pagination. This PR establishes a pattern of storing pagination state in search params so that table location is included when sharing links. I expect we'll use this pattern in most table displays.

The action menu has also been populated with options to copy data for a variable and delete variable.

Here's what it looks like:

A subsequent PR will include filtering and controlling the page size for the table.

Related to https://github.com/PrefectHQ/prefect/issues/15512