feat(server): allow listening on unix socket through server.socket config

[admosity]

This PR provides the ability to bind to a unix socket. This need came about from the default nginx build pack on heroku requiring to listen on a socket:

https://elements.heroku.com/buildpacks/heroku/heroku-buildpack-nginx#buildpack-instructions

This PR allows the instantiation of the server with the socket:

I can use server.port to achieve the same functionality, but the results are a little surprising (and it still works!):

[codecov]

Codecov Report

Merging #7824 into master will increase coverage by 5.54%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #7824      +/-   ##
==========================================
+ Coverage   27.16%   32.71%   +5.54%     
==========================================
  Files        1163     1194      +31     
  Lines       15518    12965    -2553     
  Branches     2410     1279    -1131     
==========================================
+ Hits         4216     4241      +25     
+ Misses       9534     7884    -1650     
+ Partials     1768      840     -928     

Flag	Coverage Δ
#front	24.81% <70.00%> (+5.49%)	⬆️
#unit	53.85% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...admin/admin/src/components/ContainerFluid/index.js	75.00% <ø> (+8.33%)	⬆️
...in/src/components/LeftMenu/LeftMenuFooter/index.js	80.00% <0.00%> (+13.33%)	⬆️
...rc/containers/EditViewDataManagerProvider/index.js	0.00% <ø> (ø)
...tainers/EditViewDataManagerProvider/utils/index.js	0.00% <0.00%> (ø)
.../admin/src/components/ComponentIconPicker/index.js	0.00% <ø> (ø)
...agerProvider/utils/retrieveComponentsFromSchema.js	87.50% <ø> (+18.75%)	⬆️
...ugin-upload/admin/src/components/EditForm/index.js	0.00% <ø> (ø)
...rapi-plugin-upload/admin/src/translations/index.js	0.00% <0.00%> (ø)
packages/strapi-provider-upload-local/lib/index.js	29.16% <0.00%> (ø)
...ovider/utils/getFieldsActionMatchingPermissions.js	100.00% <100.00%> (ø)
... and 1078 more

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[alexandrebodin]

Hello @admosity hank you for this, it looks good to me. We are just going to ask you to add some documentation :) I think this should be added here: https://strapi.io/documentation/v3.x/concepts/configurations.html#server

[admosity]

@alexandrebodin Sure thing! Just added documentation.

[alexandrebodin]

LGTM, Thanks !