Add support for Strimzi-native Configurable RSA Keys

[kyguy]

Type of change

• Enhancement / new feature

Description

Adds a keySize field to CertificateAuthority allowing users to configure the RSA key size for both root and leaf certificates, defaulting to 4096 bits. This addresses compliance requirements (BSI, eIDAS, NIST) that mandate RSA keys larger than the previously hardcoded 2048-bit leaf key size.

Implements: https://github.com/strimzi/proposals/blob/main/153-configurable-ca-key-size.md

Addresses: #12769

Checklist

• Update documentation
• Update CHANGELOG.md (if present)
• Reference relevant issue(s) and close them after merging
• Write tests
• Make sure all tests pass
• Try your changes inside a Kubernetes cluster, not just from unit tests
• AI assistance was used to create this PR (see the Strimzi AI policy)

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

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[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.78%. Comparing base (d8b42c6) to head (0f63fae).

Files with missing lines	Patch %	Lines
...mzi/operator/cluster/model/EntityUserOperator.java	75.00%	0 Missing and 1 partial ⚠️
...java/io/strimzi/operator/common/ca/InternalCa.java	66.66%	1 Missing ⚠️
...a/io/strimzi/operator/user/UserOperatorConfig.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #13121      +/-   ##
============================================
+ Coverage     80.75%   80.78%   +0.02%     
- Complexity     6806     6813       +7     
============================================
  Files           360      360              
  Lines         23352    23365      +13     
  Branches       3174     3176       +2     
============================================
+ Hits          18858    18875      +17     
+ Misses         3254     3253       -1     
+ Partials       1240     1237       -3     

Files with missing lines	Coverage Δ
.../main/java/io/strimzi/certs/OpenSslCertIssuer.java	81.23% <100.00%> (+0.05%)	⬆️
...n/java/io/strimzi/operator/common/ca/CaConfig.java	97.05% <100.00%> (+0.39%)	⬆️
...io/strimzi/operator/user/model/KafkaUserModel.java	84.23% <ø> (ø)
...imzi/operator/user/operator/KafkaUserOperator.java	88.66% <100.00%> (+0.07%)	⬆️
...mzi/operator/cluster/model/EntityUserOperator.java	96.03% <75.00%> (-0.69%)	⬇️
...java/io/strimzi/operator/common/ca/InternalCa.java	66.45% <66.66%> (ø)
...a/io/strimzi/operator/user/UserOperatorConfig.java	67.91% <66.66%> (-0.03%)	⬇️

... and 4 files with indirect coverage changes

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[scholzj]

@kyguy Just FYI ... the #13017 is pretty close to being merged. I think we will now aim at merging that first and then rebasing this PR on top of that. Sorry.