feat(aws-cloudfront, nextjs-component): support setting WAF web ACL id

[dphang]

Fixes: #521

Tests

Added unit tests, but also tested manually. Not including the input will leave it unchanged from whatever value it is (non-destructive), setting it to a valid ARN will update to that, and setting it to empty string ("") will remove it from the distribution (this is per AWS).

[codecov]

Codecov Report

Merging #724 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #724      +/-   ##
==========================================
+ Coverage   79.53%   79.58%   +0.04%     
==========================================
  Files          55       55              
  Lines        1774     1778       +4     
  Branches      380      382       +2     
==========================================
+ Hits         1411     1415       +4     
  Misses        305      305              
  Partials       58       58              

Impacted Files	Coverage Δ
...serverless-components/aws-cloudfront/serverless.js	78.57% <ø> (ø)
...rless-components/nextjs-component/src/component.ts	90.14% <ø> (ø)
.../serverless-components/aws-cloudfront/lib/index.js	82.92% <100.00%> (+0.87%)	⬆️

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