feat(datepicker): added customizable Button

[saschaB91]

added a Button for a customdate in DatePicker

PR Checklist

Before creating new PR, please take a look at checklist below to make sure that you've done everything that needs to be done before we can merge it.

• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated tests.
• added/updated API documentation.
• added/updated demos.

[codecov]

Codecov Report

Merging #6180 (087c719) into development (6c138db) will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@               Coverage Diff               @@
##           development    #6180      +/-   ##
===============================================
+ Coverage        77.24%   77.27%   +0.02%     
===============================================
  Files              311      311              
  Lines            10681    10690       +9     
  Branches          2625     2626       +1     
===============================================
+ Hits              8251     8261      +10     
+ Misses            2420     2416       -4     
- Partials            10       13       +3     

Impacted Files	Coverage Δ
apps/ngx-bootstrap-docs/src/ng-api-doc.ts	100.00% <ø> (ø)
src/datepicker/base/bs-datepicker-container.ts	29.54% <0.00%> (-0.69%)	⬇️
src/datepicker/themes/bs/bs-datepicker-view.html	100.00% <ø> (ø)
src/datepicker/bs-datepicker.config.ts	100.00% <100.00%> (ø)
...ker/themes/bs/bs-datepicker-container.component.ts	78.75% <100.00%> (+1.41%)	⬆️
src/chronos/i18n/pl.ts	81.66% <0.00%> (ø)
src/chronos/i18n/cs.ts	88.76% <0.00%> (+1.12%)	⬆️
src/chronos/i18n/sk.ts	86.48% <0.00%> (+1.35%)	⬆️

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[cypress]

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Test summary

87 • 0 • 6 • 0

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Run details

Project	ngx-bootstrap
Status	Passed
Commit	3475e5d ℹ️
Started	Sep 4, 2021 12:03 PM
Ended	Sep 4, 2021 12:08 PM
Duration	05:25 💡
OS	Linux Ubuntu - 20.04
Browser	Electron 87

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