Suggest using ggshield config set instance at login

[agateau-gg]

Context

To login on an instance other than the default instance, users pass the --instance INSTANCE_URL option to ggshield auth login. This does not make INSTANCE_URL the default instance though, leading to confusion.

This PR makes ggshield auth login print a message suggesting to use ggshield config set instance INSTANCE_URL when auth login is called with --instance.

What has been done

• Turn Config.set_cmdline_instance() into a cmdline_instance property
• Add the message

Demo

Logging in on the default instance, no message:

$ ggshield auth login --method token
Enter your GitGuardian API token: 
Authentication was successful.

Logging in on a custom instance, suggestion message is printed:

$ ggshield auth login --method token --instance https://example.com
Enter your GitGuardian API token: 
Authentication was successful.

To make ggshield always use this instance, run "ggshield config set instance https://example.com".

[codecov-commenter]

Codecov Report

Merging #684 (e53da9d) into main (608230e) will increase coverage by 0.04%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #684      +/-   ##
==========================================
+ Coverage   93.05%   93.09%   +0.04%     
==========================================
  Files         128      128              
  Lines        5698     5705       +7     
==========================================
+ Hits         5302     5311       +9     
+ Misses        396      394       -2     

Flag	Coverage Δ
unittests	93.09% <100.00%> (+0.04%)	⬆️

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

Files Changed	Coverage Δ
ggshield/cmd/auth/login.py	96.38% <100.00%> (+0.55%)	⬆️
ggshield/core/config/config.py	97.14% <100.00%> (+0.11%)	⬆️
ggshield/core/oauth.py	86.36% <100.00%> (-0.13%)	⬇️

... and 1 file with indirect coverage changes