add generic object exporter via annotations

[kevinpapst]

Description

Added an exporter that can generate Excel files from a list of objects, based on annotations.
No more complicated and hard-wired logic to format columns...

• Added exporter for Users - Fixes Export user list #1822 and Fixes Allow export option for Users view #1706
• Added exporter for Activities
• Added exporter for Projects - Fixes Only question: Export of all projects even if they are empty #1819
• Added exporter for Customers

Can be tested at https://demo-branch.kimai.org/

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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 #1831 into master will increase coverage by 0.03%.
The diff coverage is 95.01%.

@@             Coverage Diff              @@
##             master    #1831      +/-   ##
============================================
+ Coverage     92.54%   92.57%   +0.03%     
- Complexity     5069     5161      +92     
============================================
  Files           470      489      +19     
  Lines         15698    15978     +280     
============================================
+ Hits          14528    14792     +264     
- Misses         1170     1186      +16     

Impacted Files	Coverage Δ	Complexity Δ
src/Entity/Activity.php	100.00% <ø> (ø)	34.00 <0.00> (ø)
src/Entity/ColorTrait.php	100.00% <ø> (ø)	4.00 <0.00> (ø)
src/Entity/Customer.php	100.00% <ø> (ø)	80.00 <0.00> (ø)
src/Entity/Project.php	100.00% <ø> (ø)	62.00 <0.00> (ø)
src/Entity/User.php	98.46% <ø> (ø)	74.00 <0.00> (ø)
src/Repository/InvoiceRepository.php	69.23% <ø> (-0.39%)	21.00 <0.00> (ø)
...xport/Spreadsheet/CellFormatter/ArrayFormatter.php	85.71% <85.71%> (ø)	3.00 <3.00> (?)
...ort/Spreadsheet/CellFormatter/BooleanFormatter.php	85.71% <85.71%> (ø)	3.00 <3.00> (?)
...rt/Spreadsheet/CellFormatter/DurationFormatter.php	85.71% <85.71%> (ø)	3.00 <3.00> (?)
src/Export/Annotation/Expose.php	87.50% <87.50%> (ø)	4.00 <4.00> (?)
... and 46 more