Add Souin HTTP cache web server

[darkweak]

Thank you for taking the time to work on a PR for Awesome-Sysadmin!

To ensure your PR is dealt with swiftly please check the following:

• Your additions are Free software
• Software your are submitting is not your own, unless you have a healthy ecosystem with a few contributors (which aren't your sock puppet accounts).
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• Format your submission as follows, where Demo and Clients are optional.
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Please take some time to answer the following questions as best you can:

• Why is it awesome?

It's totally free and free forever, unlike Varnish that has an enterprise plan. It supports more RFCs than Varnish.

• Have you used it? For how long?

I maintain it.

• Is this in a personal or professional setup?

It's used in production by many companies.

• How many devices/users/services/... do you manage with it?

More than 100k users.

• Biggest pros/cons compared to other solutions?

Because it's written in go it's less efficient than Varnish. But it has more features.

• Any other comments about your use case, things you've found excellent, limitations you've encountered... ?