Add sponsorship to README

[martincostello]

• Add a sponsorship section to the README.
• Thank @aws for their sponsorship through their .NET on AWS Open Source Software Fund.
• Add .github/FUNDING.yml file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.67%. Comparing base (4f67189) to head (abd34e2).

❗ Current head abd34e2 differs from pull request most recent head 0568b8c. Consider uploading reports for the commit 0568b8c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2059   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files         312      312           
  Lines        7106     7106           
  Branches     1054     1054           
=======================================
  Hits         5946     5946           
  Misses        789      789           
  Partials      371      371           

Flag	Coverage Δ
linux	83.67% <ø> (ø)
macos	?
windows	83.67% <ø> (ø)

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