Add Feature: Footer row support in table

[venkateshsundaram]

Thanks for contributing to react-virtualized!

Description

Supporting footer row in table for react-virtualized.

Before submitting a pull request, please complete the following checklist:

• The existing test suites (npm test) all pass
• For any new features or bug fixes, both positive and negative test cases have been added
• For any new features, documentation has been added
• For any documentation changes, the text has been proofread and is clear to both experienced users and beginners.
• Format your code with prettier (yarn run prettier).
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Here is a short checklist of additional things to keep in mind before submitting:

• Please make sure your pull request description makes it very clear what you're trying to accomplish. If it's a bug fix, please also provide a failing test case (if possible). In either case, please add additional unit test coverage for your changes. :)
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Screenshots

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[venkateshsundaram]

Footer row will work accordingly based on more rows(data with scroll) and less rows (data without scroll)

Real time Usage:

Footer.Row.Virtualized.mov

[venkateshsundaram]

Hi @wuweiweiwu Can you pls help me to merge?