feat(tooltip): tooltip support get content async

[TZZack]

Plugin tooltip's getContent support async

[petercat-assistant]

Walkthrough

This pull request introduces asynchronous support for the getContent function in the tooltip plugin. The changes allow tooltip content to be fetched asynchronously, enhancing flexibility and functionality.

Changes

File	Summary
packages/g6/__tests__/demos/index.ts	Added export for pluginTooltipAsync.
packages/g6/__tests__/demos/plugin-tooltip-async.ts	New test case for async tooltip content retrieval.
packages/g6/__tests__/unit/plugins/tooltip.spec.ts	Updated tests to handle async tooltip content and added new test for async content.
packages/g6/src/plugins/tooltip.ts	Modified getContent to return a promise and updated show method to handle async content.