dpf: Improve and document data-plane-transfer

[bscholtes1A]

What this PR changes/adds

This PR improves the data-plane-transfer modules in two ways:

• it fixes a long-time TODO which was to use the data plane selector also for the Client Pull transfer.
• it finalizes the documentation of this module.

Why it does that

Solve TODO and improve documentation.

Further notes

Linked Issue(s)

Contributes to #1382

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 #1519 (5446c86) into main (cfba051) will increase coverage by 0.01%.
The diff coverage is 83.05%.

@@            Coverage Diff             @@
##             main    #1519      +/-   ##
==========================================
+ Coverage   67.12%   67.13%   +0.01%     
==========================================
  Files         736      737       +1     
  Lines       16144    16160      +16     
  Branches     1050     1055       +5     
==========================================
+ Hits        10836    10849      +13     
- Misses       4831     4833       +2     
- Partials      477      478       +1     

Impacted Files	Coverage Δ
...ataplane/flow/DataPlaneTransferFlowController.java	90.47% <ø> (ø)
...er/dataplane/sync/DataPlaneTransferSyncConfig.java	0.00% <ø> (ø)
...dataplane/sync/DataPlaneTransferSyncExtension.java	0.00% <0.00%> (ø)
...plane/sync/proxy/DataPlaneProxyTokenDecorator.java	100.00% <ø> (ø)
...xy/DataPlaneTransferProxyReferenceServiceImpl.java	100.00% <ø> (ø)
...aplane/sync/validation/ContractValidationRule.java	87.50% <ø> (ø)
...taplane/api/controller/DataFlowRequestFactory.java	100.00% <ø> (ø)
...ctor/dataplane/api/response/ResponseFunctions.java	83.33% <ø> (ø)
...oxy/DataPlaneTransferConsumerProxyTransformer.java	89.28% <71.42%> (-6.72%)	⬇️
...ataplane/api/validation/TokenValidationClient.java	72.22% <71.42%> (ø)
... and 6 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cfba051...5446c86. Read the comment docs.