feat(response): add link-extension to append_link()

[kaichan1201]

Summary of Changes

This PR adds link-extension to append_link() as specified in RFC 8288 Sec. 3.4.2. From my understanding, there is not any restriction on what the extension attributes could be, so I implement this feature in the most intuitive way: having the user pass in pairs of (parameter, value) and concatenate them onto the header.

Related Issues

Closes #288

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[codecov[bot]]

Codecov Report

Merging #2055 (5c8e64f) into master (05d11a3) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2055   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           62        62           
  Lines         6743      6746    +3     
  Branches      1253      1255    +2     
=========================================
+ Hits          6743      6746    +3     

Impacted Files	Coverage Δ
falcon/response.py	100.00% <100.00%> (ø)

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