Add flag to always authenticate when cloning public repositories

[LamaAni]

As a developer using an custom git server (e.g. Github Enterprise) I would like to be able to authenticate
the user on repositories which are marked as public.

See issue: #473

Ref: #693 (comment)

[6543]

@LamaAni can you add it to the documentation?

https://woodpecker-ci.org/docs/administration/server-config (https://github.com/woodpecker-ci/woodpecker/blob/master/docs/docs/30-administration/10-server-config.md)

[codecov-commenter]

Codecov Report

Merging #760 (656af2e) into master (5f79739) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 656af2e differs from pull request most recent head c92c412. Consider uploading reports for the commit c92c412 to get more accurate results

@@           Coverage Diff           @@
##           master     #760   +/-   ##
=======================================
  Coverage   52.23%   52.23%           
=======================================
  Files          86       86           
  Lines        5940     5940           
=======================================
  Hits         3103     3103           
  Misses       2665     2665           
  Partials      172      172           

Impacted Files	Coverage Δ
server/shared/procBuilder.go	86.14% <100.00%> (ø)

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[LamaAni]

@LamaAni can you add it to the documentation?

https://woodpecker-ci.org/docs/administration/server-config (https://github.com/woodpecker-ci/woodpecker/blob/master/docs/docs/30-administration/10-server-config.md)

Sure thing. Updated