CI: add checkstyle rule for StaticVariableName

[algattik]

What this PR changes/adds

Add a rule for checkstyle check StaticVariableName.
Checks that static, non-final variable names conform to a specified pattern.

Why it does that

Improve code consistency and reduce spurious PR diffs.

Further notes

Some fixes involved making variables final that are used as final.

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 #1165 (db126d2) into main (045c9a6) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##               main    #1165   +/-   ##
=========================================
  Coverage     62.65%   62.65%           
  Complexity     2981     2981           
=========================================
  Files           694      694           
  Lines         15428    15428           
  Branches       1048     1048           
=========================================
+ Hits           9666     9667    +1     
  Misses         5331     5331           
+ Partials        431      430    -1     

Impacted Files	Coverage Δ
...taspaceconnector/boot/monitor/MonitorProvider.java	0.00% <0.00%> (ø)
.../dataspaceconnector/boot/util/TopologicalSort.java	95.23% <0.00%> (+2.38%)	⬆️

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