saml: allow to keep existing roles on login

[kevinpapst]

Description

new SAML configuration to keep existing user roles upon login

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #3440 (5edda14) into master (8d695d0) will decrease coverage by 0.02%.
The diff coverage is 76.74%.

@@             Coverage Diff              @@
##             master    #3440      +/-   ##
============================================
- Coverage     91.80%   91.77%   -0.03%     
- Complexity     7845     7861      +16     
============================================
  Files           742      742              
  Lines         23682    23719      +37     
============================================
+ Hits          21741    21768      +27     
- Misses         1941     1951      +10     

Impacted Files	Coverage Δ
src/Configuration/SamlConfiguration.php	87.50% <0.00%> (-12.50%)	⬇️
src/Configuration/SystemConfiguration.php	97.41% <0.00%> (-1.28%)	⬇️
src/Validator/Constraints/TimesheetBasic.php	100.00% <ø> (ø)
.../Validator/Constraints/TimesheetBasicValidator.php	95.19% <16.66%> (-4.81%)	⬇️
src/Saml/User/SamlUserFactory.php	96.42% <75.00%> (-1.69%)	⬇️
src/DependencyInjection/Configuration.php	97.72% <100.00%> (+<0.01%)	⬆️
src/Entity/Project.php	100.00% <100.00%> (ø)
src/Form/ProjectEditForm.php	100.00% <100.00%> (ø)
src/Repository/ActivityRepository.php	91.74% <100.00%> (+0.69%)	⬆️