CI: checkstyle unused imports rule

[algattik]

What this PR changes/adds

Add a rule to fail build on unused imports.

Why it does that

Remove unnecessary coupling and dependencies, and reduce PR diffs.

Further notes

Linked Issue(s)

Relates to #991

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Merging #1011 (050e088) into main (2a1db74) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1011   +/-   ##
=========================================
  Coverage     62.61%   62.61%           
  Complexity     2969     2969           
=========================================
  Files           694      694           
  Lines         15416    15416           
  Branches       1044     1044           
=========================================
  Hits           9652     9652           
  Misses         5334     5334           
  Partials        430      430           

Impacted Files	Coverage Δ
.../dataspaceconnector/common/string/StringUtils.java	83.33% <ø> (ø)
...paceconnector/boot/system/runtime/BaseRuntime.java	0.00% <ø> (ø)
...pi/multipart/handler/ContractAgreementHandler.java	0.00% <ø> (ø)
...ds/api/multipart/handler/ContractOfferHandler.java	0.00% <ø> (ø)
...pi/multipart/handler/ContractRejectionHandler.java	0.00% <ø> (ø)
.../ids/api/multipart/handler/DescriptionHandler.java	46.80% <ø> (ø)
.../multipart/handler/NotificationMessageHandler.java	93.75% <ø> (ø)
...description/ArtifactDescriptionRequestHandler.java	0.00% <ø> (ø)
...cription/DataCatalogDescriptionRequestHandler.java	0.00% <ø> (ø)
...ption/RepresentationDescriptionRequestHandler.java	100.00% <ø> (ø)
... and 14 more

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