[Feature Request] How to configure a bottom border with a width of 2px for the merged cells at the of a table?

### Existing Component
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### Component Name
Table表格

### Description
I want to customize the width of the bottom border of a table, but it is not currently possible to achieve this through configuration. Will there be support for configuring the width of all four borders in the future?

Start by reviewing the Table component's existing styling and configuration entry points for border behavior. Determine how a configurable bottom border for merged cells, and potentially all four borders, should fit the component API. Done means the supported configuration is documented and verified for the requested table case.

Written by the indexing model from the issue text.